I added another experimental $1 silver certificate to my collection, this one from the series 1935 CB block. PMG pop is 4 in 66EPQ with 2 higher (both 67EPQ).

There were several different versions of experimental $1 silver certificates printed to test different paper compositions.
The best known are series 1935-A notes from the SC block with a large red "R" and "S" on the face, with one being regular paper (the control) and the other being a special paper composition. Not surprisingly, this test had some issues as people noticed the big red letters on the face and many were quickly pulled from circulation.
Both series 1928-A and 1928-B had experimental notes from the XB, YB and ZB blocks. These can be identified, as were the other versions of experimental notes, only by the serial number ranges. The XB and YB notes were different paper compositions and the ZB notes were the control.
The series 1935 experimental notes were from the AB, BB and CB blocks. For these, the AB notes had a special finish, the BB notes had special paper, and the CB notes were the control.
Lastly, series 1935-A had experimental notes from the XB, YB and ZB blocks. The XB notes were control and the YB and ZB notes were of different paper compositions.
Here's are a couple in-depth articles about these notes if you'd like additional info:
https://www.pmgnotes.com/news/artic...ental-Notes/https://www.numismaticnews.net/arch...xperimentals
Member of SPMC, FCCB,
ANA and ANS.
My U.S. Classic Commemorative Complete Set:
https://www.NGCcoin.com/registry/co...sets/278741/