Why has there never been a description or analysis of the watermarks USPS and USIR? When I think about watermarks of other countries, little differences has been made to define a different watermark. And in this case it's even clear that it is a different watermark. I searched for a while in the internet to find a good picture of an USPS watermark (some stamps in a block), but did not find it (the best is on stampsmarter but not that good as the USIR example). So the first step would be to create good images of USPS and USIR watermarks. I guess not many collectors have seen those. The next step would be quite easy, to compare the 2 watermarks in image software. So is there a good image of an USPS and USIR watermark (good image in high resolution)?
I know that there are plastic overlays of the USPS single and double line watermarks. I don't recall seeing one for USIR. It's really the I that is more of a potential problem, either the leg of the R is there or it is not. Most watermarks from that issue are fairly obvious, so the number of problematic watermarks is fairly small.
"Can you point to any copies that have a cert and have a partial letter"?
Just go to the PF site and look it up. Any cert that does not specifically state that it's a full letter does not have to be one. It is beyond a reasonable doubt to think that EVERY cert over 15 pages that does not state specifically that it is a full letter actually has one.