What I like most about this stamp community is that there are so many experts in so many fields.
Here is the Folklore issue of 1990 with the rarer perforation 12.5 x 12.0:
I can visualize how many stamp varieties originate (colour, shifts, scratches, paper) but I can't think of how a change in perforation would occur.
Does somebody rush in, mid-production, and say "Stop the presses! We're using the wrong perforation"?
or "This 13.3 perforator is getting dull, but all we've got is this old 12.0 one"?
I'm sure there are some of you out there who know the answer to this question that has bugged me for quite a while.