Thanks Ken, I think you are right, though as I recall, this suffix is not always applied. When I learned of this (
Storz I believe also reports this? Which means he found a document/protocol in the archives, for Storz knows next to nothing about the study of actual rifles...), I discovered that some buttplate lacked the addition of this suffix.
Unfortunately, just how consistent/common this observation is will be difficult to determine because so few show this angle. For example, i just searched through about a hundred rifles by Mauser late 1917-1918 and not a single rifle shows this location. That as a statistic is many times worse than the observation of Imperial barrel coding, which about half a dozen have been observed!
Speaking of barrel coding, hopefully the OP will get a chance to observe both this buttplate suffix location and the barrel code, it would be good to see if his buttplate shows a "cc" suffix, which would be a good sign that this practice was in use late 1917 through 1918. The BC will undoubtedly be "BS" (Bergische Stahl-Industrie-Gesellschaft Remscheid) which seems to have the sole contract for Mauser barrel blanks and possibly all metal forgings. Mauser also stamped the BC near the receiver on the underside with the appropriate acceptance stamps, not the practice used at the Berlin firms (DWM/Spandau/Oberspree) in front of the RS (which varied by range)
*** I would expect that if this practice, the suffix, was in consistent use, especially late in the war, that this would be best proven by observations at Mauser or Amberg, they were the only high quality makers late in the war, both retaining strong consistency throughout the war. This buttplate suffix question of consistency is a good subject for further study, something I will try to keep track of in my trends work.
The 3 Imperial Gewehrs I have handy all have the suffix letter on the top of the buttplate, above the upper screw. IIRC, this was not done with Imperial Karabiners.
Edit to add image of the buttplate of 1916 Amberg 3651p.
