I see a "ES" just before the forend lug, but can you confirm that the letters "RM" are just ahead of the forward barrel lug? It's definitely not by Sauer and my first guess would be E. Schmidt & Habermann but let me check my notes as I've seen a similar frame/action. It was made between 1912 & 1923 and all the tubes of Krupp steel were forged at G. Schilling's facility. One of the Schillings might have made it but if the initials are "RM" it might take us to one of the Zella-Mehlis craftsmen.

Kind Regards,

Raimey
rse