The 20h, Scott 45a was issued with two perforations: 13-3/4 all around by line perforator, and 13-3/4 x 13-1/2 by a comb perforator.
The example in the link from perf12 shows a typical shift of the comb device downward, creating wider margins than normal at the top and bottom. The width of course cannot vary due to the nature of the comb perforator. This DOES allow trimming to make an imperf stamp, though the left and right margins cannot be trimmed too much. A perforated example would be needed for comparison and measuring.
If the settings of the perforation wheels on the line perforated stamps are quite wide in some places, the margins could vary considerably, which could allow for trimming and still maintaining decent margins. This can happen on all four sides.
When I am home I will see what I have in my collection.
Excellent information here:
http://www.knihtisk.org/hradcany/20...1V5N14_0.pdf