|What follows makes a comma look like an event of geological significance.
What has gone before leads me to the whole area of issue dates. There are a number of articles which deal with this matter on the website. The gestation period for a set of cards can be considerable. Work is still needed to accurately date some issues but for the vast bulk of cards we know the year of issue and in most instances the month they were first put into packets. Happily though for people like me in some instances things are just not that simple. The first World War caused plenty of problems to many aspects of life. During that time a number of cigarette card issues were withheld and issued later. Players, Cries of London 2nd series is a case in point. There is evidence that this set was first issued in 1916 but the bulk of the issue appeared in 1922.
Murray's lists the set with an issue date of 1916, but does note there are two distinct back variations, blue back and black back. The black back being 3 times more expensive. It would be easy to assume the black back printing were the cards issued in 1916 and the blue back 1922 but it is more complex than that (natch). The black back cards were not in fact issued at all.
Do not get the idea Murray's chooses the earlier date as a matter of course. The same sort of thing happened to Players, Minatures issued first Dec.1916 and then re-issued Jun.1923. Murray's lists them as a 1923 issue. Just to prove the rule about disruption from the War by breaking the rule, Players, Shakespearian series was first issued in 1914 and then 1916. (Murrays has a catalogue date of 1917).
To attempt to differentiate between some of these 're-issue' sets is basically impossible and you might as well try to determine what day of the week a card was taken out of its carton. It is probably about time we stopped dancing on the head of this pin, it is getting crowded.
Players were not the only manufacturers doing this type of thing they all were to greater or lesser extent. If you look at any guide for cigarette cards you will see some sets have cards which are much rarer than the rest, making the set rather tricky to complete in some instances. Churchmans produced a set called Fishes of the World which was issued as a series of 50 cards in Sept.1911. Then in 1924 they re-issued the set but for some reason only printed 30 of the 50 cards. This makes a complete set of 50 cards difficult to obtain. The limited supply of particular cards is fortunately not common and this Churchmans example might be the most extreme in terms of the range of numbers in short supply. The whole business of sets with cards which have been withdrawn or sets with additional subjects amended to them is a whole different area of study (and mighty interesting it is too, honestly). Now if you thought punctuation and colour variation was quite enough hair splitting for one day perhaps you should not read on. Only those with a strong grasp on reality should venture further. What follows makes a comma look like an event of geological significance.
For those stout of mind who are venturing further a simple test of nerve. Churchmans, East Suffolk Churches  black front (not sepia mind) was printed on two slightly different color of card. It is described as cream back or white back. I would suggest these descriptions might be over-emphasizing the difference. Ninety years later most people are happy not to worry about this difference. Mind you, Players, Freshwater Fishes was issued on two different coloured cards and these are noticably different, although I think it is the adhesive on the back of these 'sticky-backs' rather than the card. In 1933 the set was issued and has become known as the 'pink' back and then in 1934 it was issued again but this time as the more normal white back. The difference is obvious and to try to amalgamate the two sets would not be right at all, especially as there is no economic necessity to do so. Why is one set pink? Don't quote me but I think this is the first ever set of 'sticky-backs' produced, perhaps they were just getting the glue right.
There are other instances of colour variation in the card (as opposed to ink) but quite honestly it messes with my head just thinking about it so I am not going to. For the most part it matters not a jot and there would be more difference in the colour if you looked at the card under a 40 watt bulb or a 100 watt bulb. In fact a lot more difference.
That is not to say card is not important, Carreras produced the Alice in Wonderland set in 1930 and for whatever reason a set of cards can be completed with straight corners and another set can be completed with rounded corners now pretending that difference is not important would be too much for just about everyone.
So far I have only engaged your sense of sight when determining variations in card. The next sequence of variation is best determined by touch, more sensitive than sight in a number of ways. Card thickness is what we are talking about here, but don't think this is something you are going to see just by looking at the card, although possible it is not a very accurate way of doing things, there really is not enough difference between the two extremes. A micrometer will determine the difference (yes I use micrometers). I was going to dismiss this variation as almost unnecessary even to mention but changed my mind for a number of reasons. Players produced a set, Products of the World in 1908 [Murray's date]. Another source identifies a thick card version and a thin card version. It goes further and says the thin card was issued in Feb.1908 and the thick card May 1909. The only sure fire way of knowing which is which is to bend the card. Thin card bends more. This is not something to do with a heavy hand but lets not get to precious about cards. I could show you an animated gif of me bending a card but it might be too shocking on so many levels.
Just because there are differences in the thickness of cards does not mean there were differences in the date of issue. It can simply mean they fed different thickness of card into the machine at the time of going to press. We all know how business likes to save a few pence and reducing the thickness of card is one way of doing just that. Sets like Players, Historic Ships  or Wooden Walls  seem simply to have various distinct thickness of card. To make differentiate these sets is just to drive the price up.
Before I leave the topic of card thickness Players, Riders of the World has a thick and thin variation. The thick card was probably issued in 1905 and the thin card issued in 1914.
Murrays does not note the difference but before we take up burning torches and rush to storm the gates there are swings and roundabouts to all this. I use Murray's catalogue as the starting point to discount the cards I sell. I could use catalogues which recognise some of these fine variations in cards but there is a price to pay (literally) for this greater level of accuracy.
The simple rule of demand and supply. If 50% of the cards are thin board and 50% are thick board then differentiating has the potential of doubling the price. Also put into the equation those determined to collect all variations and there can even be an increase in demand combined with a restriction in supply.
You might have gathered by now I could go on and on with this subject, it is almost endless because you can mix and match these variations and more, you can have cards printed on a different type of board with and without comma's.
Remember though significant card variation is reasonably rare and rarer still that it actually makes any real difference to things. You pays your money and makes your choice, it is just useful to know you have a choice.