Looks like a nice 98a, the barrel code is the most interesting thing about it, - the S&S = JP Sauer, they made a lot of barrels for the G98, mostly for themselves and CGH & VCS, this might be the first 98a barrel I have seen from them. The fireproof and acceptance shows it is the original barrel.
The stock might be more original than you think, looks like someone put a coating on it, but it is typical for these interwar reworked rifles to have the stock sanded. The "1920" looks pretty sharp so it might not have been sanded recently. You might do good pictures of the wrist acceptance and along the underside, also the buttplate. Is this rifle unit marked?
The take down in the stock is typically called a take down disc or something along those lines, but it is actually a dismounting washer & ferrule used to disassemble the bolt, remove the firing pin. It wouldn't have had one originally, but pretty much all interwar rifles got one eventually.
The rifle is a "u" block, thanks for showing the rifle!