The Contract Assassins stamp, issued in the Year of the Reversed Ptarmigan, was Discworlds first diamond shaped stamp. It still features the arms, but with virtually black borders it has a sinister look to it, quite appropriate to the guild. However the effect is spoilt somewhat on single stamps as the white paper can be seen at the torn perforations, so have a rather ragged look. The diamond shape is accommodated by pacing the stamps at 45 degrees in the sheets. There were no sports, varieties or errors reported with this stamp.

An imperforate test print sheet shows what is probably a work-in-progress stamp. The background is plain rather than chequer-board and without a box around the 3p. The lighter elements have a pale mauve wash, against the creamer colouring used on the issued stamp. This may be a test print to see how well the main elements look against a dark background, or indeed a rejected design. The sheet make up and decoration is virtually identical with the issued stamps (although the T&S logo is much larger and a larger font was used), suggesting that it appeared late in the development of the stamp.
Addendum - some perforated examples of singles stamps have appeared in mixed Discworld stamp mixtures sold by on-line auction.

Addendum 2. A colour test image of a computer file shows that quite a range of subtle colour variations for this stamp were prepared. Interestingly, the chequer-board background is missing from a number of the options, and the stamp above is perhaps that on the top left. I have heard that these were just an over-inked aberration, but this sheet indicates that a more solid background was deliberate, and that some were not only printed, but sheets were perforated. Again, this image is copyright Colin Edwards and Alan Batley.