I mostly agree with Old Guns and I too I believe the fore end is original and the butt stock has been replaced. Typically the checkering on the fore end is very worn, often nearly completely off while the checkering at the grip is frequently in quite good nick. I have two in such condition The profile of the pistol grip is entirely wrong for a drilling from that era and is devoid of any signs of having ever been checkered from what I can see in the picture.
I can't make out the proofs nor am I smart enough to know how to enlarge them. If there is a "118/35" on the rifle barrel it is most likely the 9.3 X 72R but, there was 4 different cartridges carrying that description. Three are very similar in that they are straight taper cases but, usually not quite interchangeable. The 4th is a bottleneck Sauer & Sohn cartridge case and not interchangeable with the other three. Only way to know is cast the chamber and if you're smart do it before you buy ammo. If the word "Nitro" is in block letters it was proofed in Suhl, if in script, Zella-Mehlis.
Old Guns is right, there is no model name or number. It is a drilling....period. Unfortunately it has been re-blued, you're both correct.
For the era, for a drilling, I would call your guns' quality typical. Keep in mind that the "typical" drilling was a very finely made hunting gun so that isn't to call it run of the mill in the overall scheme of things. Simpson was a long time player in the firearms industry but whether they actually made yours is open to question. Many were sourced "for the trade" from Suhl and Zella-Mehlis with the retailers name on the rib.
Sadly, no sir, it is not a "diamond in the rough". As a using drilling it's excellent but there has been too much done to it for it to have any more than utilitarian value. I have a few drillings and am always on the lookout for another. One such as yours it seems to me $1000 would be absolute tops from anyone who knows what he's looking at. One sees a lot more asked for them but as Old Guns described, they usually languish on a rack or gun auction site. It's a pretty small market one is playing to. Those of us who like them, love them but, as with all collectors of anything, we want them original.