Comparing my genuine stamps (left column) with the fakes in my collection (right column) I noticed that the two mint copies I have of both stamps each belong to one of two slightly different varieties. The differences aren't big enough to call them setting varieties, but anyway. Compared with the stamps at pos. 1 and 3, the two copies at 5 and 7 have the "c" shifted to the right, and the "t" is also different, with a characteristic flaw (dent) in the hook. It almost looks like the forger of no. 6 tried to copy that flaw. This is probably easier to see in the image below with the masked overprints.
Incidentally, my three used copies are all of the first type. Does anyone have multiples of #156 or #O91?