The USIR watermark is very similar to the double-line USPS. Here is a normal 832 on top of a Special Tax Stamp printed on the USIR paper. You are correct that any 832b stamp may have a full letter or only small bits of the letters depending on how it fell on the paper.

(I do not know if the orientation should be horizontal or vertical,so I may have the stamp 90 degrees off, but I'm sure someone who knows will post shortly!)