PART ONE of the Stanley Gibbons' GB Queen Victoria Specialised Catalogue lists a number of stamps as having the variety "Double Perforation".
Though they do describe it, as looking like the sides are rouletted, I am a great believer in a picture being worth a thousand words.
So, here are a couple of examples. A penny red (plate 148) lettered AB and a 3d plate 5 (used 1869 in Malta) lettered AF. Both have faults, but will do for illustration purposes.
There is less than a 1% chance of them both being A-row, so that is probably significant.
Perhaps they are a variant of the "Long A row", where an adjustment of position is needed at the beginning of a perforation run.
A movement of 1 perforation could produce a long (or short) row at the top, or bottom depending on which way up they were.
Half, or one and a half perforation movement would produce this. The bottom of the 1d seems to be missing the bottom vertical perforation entirely, the 3d a bit less.