All offset stamps are identified by plate number. Only one Type of the 2c Offset stamp is shown imperforate since all 5 Types are shown in the sheet stamps and they all come from the same plates. There were no Type variations in the 1c Offset stamp, and only the Type IV 3c Offset stamp was issued imperforate, but plate singles are used to differentiate them from Flat Plate Press stamps.
Most of the stamps printed on the Rotary Press were intended as coils. Because of that, plate numbers are not an option. The fact that the image is stretched in one dimension on a Rotary Press is the key to identifying these stamps.
First is the 2c imperforate coil (459). A simple measurement of the width of the design (19.5 mm) confirms this is an imperforate coil and not a Flat Plate imperforate.
There was one experimental 3rd Bureau sheet stamp printed on the Rotary Press. That is the 1c stamp, catalog 542 (perf 10 x 11) or 543 (perf 10). These stamps differ only in the perforation, so measurements of the height (22.5 mm) and the perforation (10) confirm this stamp as 543. (The same stamp perf 11 is simply sheet waste perforated on the Flat Plate perforators).
Everything else in 3rd Bureau Rotary Press is coils and coil waste. All coil waste is from sidewise coils, so the image is stretched sideways (19.5 mm). Every 3rd Bureau stamp perforated on all 4 sides that is 19.5 mm wide is a coil waste stamp. There are two perforation varieties of the 1c stamp, so in addition to measuring the image width, a perforation measurement is also needed. There are actually two Type varieties on the 2c coil waste stamp, but all 3 types of the 2c coil are documented in both sidewise coils and endwise coils, so only a single example of the 2c coil waste is shown. That’s why Type III is documented in addition to the width and perforations. The 3c coil waste doesn’t have any perforation or Type varieties, so simply measuring the width of the image is sufficient.
Coils: Identifying 1c coils is theoretically easy; just measure the image (22.5 mm tall for endwise coils or 19.5 mm wide for sidewise coils) and check for a watermark. But I must admit it’s difficult to definitively say a stamp is unwatermarked instead of single line watermarked. This problem haunts the rest of 3rd Bureau Rotary Press coils.
The 2c endwise coils and sidewise coils both have 3 different Types. I have Certificates of Authenticity for the Type I endwise and the Type I and Type II sidewise coils. The other Types are self-certified.
The 3c sidewise coils have Type I and Type II self-certified. A watermark check is needed for 3c Type I and the 4c and 5c coils. The 3c endwise Rotary Press coil and the 10c Franklin sidewise coils are easy since there were no Flat Plate versions of these stamps.