The location of the stamp is most usually where this R.S. stamp is - just ahead of the barrel flats. Other initials are also commonly found include E.E. and a few others. Some, including me, have hypothesized that they represent the barrel man's or finishers mark. It seems a logicial location for such a mark and the intiials vary over time indicating more than one person doing this job. But again, this is just a guess.
Ken
The "E" gunsmiths are also on a short list. I would guess Emil Eckoldt to be the choice but later his products were marked "ECKO". He did however, have an "EES" or "EES" mark in a heart for Emil Eckoldt Suhl. But in the 1920s Emil's sons Emil(Jr.??) & Hugo had a some sort of relationship with Robert L. Schilling. I've also put some stock in the initials on the tubes near the flats with a tube maker but Emil Eckoldt for example doesn't seem to have been connected with tube making, forging or metal drawing. Being pure conjecture, but he could have had an earlier relationship with Schilling, but I would expect to see a Schilling mark on the tubes. So I'm not sure if the initials are for the tube maker, finisher or both or it could be for the contractor who sourced the components and provided it in the white to a firearms merchant. Now all this hinges on the initials being of German origin even though the tubes may have been sourced from Belgium. But some of the fellas listed as gunsmiths, or providing gunsmithing services, say in the 1880s, didn't obtain the rank of master until post WWI(maybe it was a son of the same name?). So it's still a huge puzzle with many missing pieces.
For now I guess the "HUF" to be Helfricht & Fischer of Zella St. Blasii who began circa 1860.
Kind Regards,
Raimey
rse