From about 1910 to about 1930, stamps were issued with various perforation sizes: perf 10, 11, and 11 x 10.5. These are mainly the Washington-Franklin heads and the regular issues of 1922 (beginning with Scott #551), or what collectors often refer to as the Fourth Bureau Issue. Why the different perforation sizes?
The 1939 Stamp Specialist includes an article from philatelic scholar and researcher, Max Johl, that sheds some light on the situation as the Bureau of Engraving and Printing (BEP) transitioned from flat plate printed stamps to stamps produced by the rotary printing press.
Flat plate stamps were produced one sheet at a time by flat printing plates. Each sheet was produced separately and also required the operator to gum and perforate each sheet separately. It was slow and labor intensive.
B. F. Stickney was a mechanical expert for the BEP at the time. Stickney designed an experimental printing press that printed stamps using long, continuous rolls of paper. The press used two printing plates, attached to a round drum. Each plate was curved for mounting to the printing press. That's why rotary press stamps are slightly wider in one direction than flat plate printed stamps. The curving of the printing plates caused the stamp design to "stretch" slightly in the direction of the curvature.
The BEP experimented with different perforation gauges until they found a suitable size. Stamps that were perforated 11 had a tendency to crack along the vertical perforations. Stocks of stamps soon turned into vertical strips of stamps and postal workers despised them because they were hard to count and inventory. The perforation holes left too little paper in between, causing the perforations to break apart.
Perforated 10 sheet stamps were difficult to separate. There was too much paper left in between the holes and they were hard to separate. That's why perf 10 stamps are scarce to find with fully intact perforations. Many times, the perforations are ragged or one or more perforations are shorter than the rest. The public despised the perf 10 issues.
However, perf 10 stamps were perfect for coil stamps that were used in stamp affixing machines. The perforations were strong enough that the rolls of stamps didn't separate inside of the affixing machines, but yet just weak enough for the stamps to separate for affixing to the envelope.
The BEP finally switched to perf 11 by 10.5 stamps which solved the problem. The perf 11 horizontal perforations were never a problem. The new 10.5 vertical perforations were just right. The stamps didn't fall apart into strips like the all perf 11 stamps. And the public had an easier time separating the stamps than the perf 10 issues. The 10.5 perforations left just the right amount of paper between the holes.
In a nutshell, that's why the different perforations were used. Yes, I neglected some of the details and I intentionally left out some of the odd size stamps like some of the coil waste issues. This was only meant to give you a general overview of why the BEP kept changing perforations on stamps until they found a suitable size.
You can find a reprint of Johl's article in the Sept 4, 2009 issue of Mekeel's & Stamps magazine. If you're not a subscriber, you should be. Contact Stamp News, 42 Sentry Way, Merrimack, NH 03054 or Subs@StampNewsNow.com for subscription information.