The first Ramtop stamps, celebrating Summer and Winter, had quite a long run. They overlapped with the later Autumn and Spring issue. However, there was a good reason for this. Apparently instead of an order for the new stamps going to the printers, the order was for the original stamps. So instead of the two issues running consecutively they ended up concurrent. This means that the Summer Winter stamps had multiple prints. In fact my eyes can see four different depths of colour of the stamps. Because the sheets were made up of alternating Summer and Winter stamps this is reflected not only on both Summer and Winter stamps, but also that sports of the four intensities also exist. Here they are shown from palest to brightest.

Close up images show that these differences are not associated with colour registration, but are caused by intensity of the dot printing. Compare the details from the palest and darkest of each stamp. The patterns are very comparable in all ways apart from the amount of ink deposited on the paper.

There is more.
Two different papers were used for the stamps. These can be identified by the gums. One is shinier, while the other has more of a matt appearance - not a lot in it but the difference can be seen. The shiny gummed paper was that in use originally, and the matt gummed paper came into use later, perhaps around the time that the Autumn and Spring Ramtop stamps were released, as these use the same paper. All four shade variations exist with shiny gum, while I have only found the ‘slightly pale’ and the ‘deepest’ print shades with matt gummed paper.
So that is six variations of each stamp, plus the sports, to find room for in your collections.

One further oddity in my collection is a horizontal pair imperforate between the stamps. From the sheet arrangement there would be such nine pairs if only one sheet affected. Four of these would have either Winter or Summer as the left hand stamp, and there would be five with the opposite arrangement. This must have been noticed at The Discworld Emporium when sheets to fill the LBEs were torn up. So was the error included to add something extra to a few collector’s collections, or were they being parsimonious, not wishing to bin 18 stamps?