Some US issues were printed by both flat plate and rotary printing presses. Many of these were stamps from the 1920s when the Bureau of Engraving and Printing was transitioning from the older flat plate printing presses to the newer rotary printing presses.
How do you tell the stamps apart?
If you have a stamp (or multiple) and the selvedge with the plate number is still attached, it’s very easy. On flat plate printed stamps, plate numbers are in the center of the printing plate, on the sides and/or top and bottom of the plate. The plate numbers are centered within the selvedge.
On rotary press printed stamps, plate numbers are always in one of the four corners of the printing plate. The plate number is aligned to one side of the selvedge. Rotary press plate numbers always start or end right next to a row of perforations.
Not all stamps are going to have a plate number attached somewhere. What’s next?
Depending on the stamp issue, other characteristics change between flat plate and rotary plate issues. For example, there could be a change in perforation or watermark that distinguishes one stamp from the next.
We’re down to the last resort. We have two stamps that share the same characteristics. How do you tell them apart?
Here is an important fact to know. The same dies that were used to produce the flat plate printed stamps were used to produce the rotary press printed stamps.
For flat plate printed stamps, the printing plate was, of course, flat. It printed one sheet of stamps at a time, typically 200 or 400 stamps.
For rotary press printing plates, the printing plate was created when it was flat. Before printing stamps though, the printing plate was curved so that it fit on the printing press. This curvature results in stamps that are slightly larger than their flat plate counterparts. Rotary press stamps are about one-half to one millimeter larger in one direction.
For vertically oriented stamps (ex, Scott #632), rotary press printed stamps are taller than their flat plate counterparts. For horizontally oriented stamps (ex, Scott #701), rotary press printed stamps are wider than their flat plate counterparts.
Here is another tip. Take a known flat plate stamp and a known rotary press stamp. Use damaged copies of inexpensive stamps (such as #553 which is flat plate and perf 11 and Scott #632 which is rotary and perf 11 x 10.5). Cut the four corners out of each stamp as illustrated here.. They are worthless, damaged copies anyway, so you’re not destroying anything of value.
Now you can lay one of these stamps, with the corners removed, on top of a stamp in question. The design of the stamp in question will line up with one of these two stamps. You may need a magnifying glass to see the difference. I find this to be easier than using a millimeter scale.