I just learned about this forum (courtesy of the NRA) and will comment on your S-code question. Better late than never, hopefully.
I have two S-codes-- the lower number, '1103 a', has the "S" at the rear of the toggle. This gun is completely correct, down to the magazine-- but the magazine is numbered '1103' and '+' only. However, the E-6 inspection stamps (2)are correct, and the chance of having a magazine of the same number but from an earlier gun seem to me to be remote. So my first question is-- was the suffix commonly used in numbering S-toggle magazines? Or, like elsewhere, was it omitted?
The second gun is very interesting. It has an S-toggle, but with the "S" centered in the toggle. The gun's number is '1864 a'; I realise that this is considered to be above the range of S-toggle Simsons. However, the E-6 stamps are where they should be on all parts, except for the barrel. The barrel is unmarked except for what looks like an E/N commercial proof on the forward base of the front sight. It is 9 mm. There is no S/N.
The left side of the receiver, over the serial number, very definitely is stamped with E/N, which I believe was the commercial nitro proof-- but in Nazi days. Additionally, the font used to number this gun is slightly smaller than the font used on '1103 a'. It matches the height of numbers on contemporary Mauser guns. The four Simson Army acceptance stamps are in place on the right of the receiver, though.
And the magazine is correct Simson, but with a S/N about 200 numbers earlier than the gun (and no suffix letter).
So, who made it? Simson made the parts, or at least most of them; could Mauser have gotten Simson's old work-in process and made some commercial guns to use up the inventory? Or was this, perhaps, made during the 'Wilhelm Gustloff-Stiftung' era, to use up parts?
Anyone have any ideas?