I would understand that meant
Perf
10
11½
10 x 11½
11½ x 10
I have never seen this explained, as to why this happens,
You would think that if it is a single line perforator,
one would just swivel the sheet, and perforate.
I know one Australian stamp issue, was directed "18 holes to the inch"
On the single line perforator, it was guaged Perf 14.2, on the Comb machine
it is guaged 14.25
You can identify if the stamp was comb perforated by matching the edges
to a known stamp guage, when different, the holes get progressively unmatched.
Perforating early stamps (apologies old image, and became split somehow)
