These Tell-type single shot actions were not only made by Sauer&Sohn, Suhl, but were also a staple of the Suhl/Zella-Mehlis guntrade. In fact, most of the examples I have encountered are of Zella-Mehlis make. A photo of the proofmarks would be helpful in determining the origin, but there is already a hint to the origin: If the word "NITRO" is in block letters, the gun was proofed in Suhl. If "Nitro" is in script, it was proofed by the Zella-Mehlis proofhouse and was certainly not made by S&S.