Regarding your proof marks:

The +2 and -2 stamps are indications of chamber depth that were used at the Steyr factory to match depth of threads in receiver to barrel. My M1910 Take Down Model bears '+15 and -15' stamps.

Your '883.20' stamp indicates the 883rd arm to be Nitro Proofed during calendar year 1920 at the Vienna Proof House which is indicated by the NPv stamp.

The 2860 is your stutzen's serial number from Osterr Waffenfabrik Ges. Steyr and you should find matching numbers stamped on underside of bolt handle as well as on stock under the barrel.

The 'T in circle' indicates Tiegellgusstahl, a high quality 'crucible steel' processed in the Steyr region, the 'V in circle' is also a mark from OWGS.

As you may already know, the 'Cordite 43 - 270 MAX' is a British mark indicating a proof load of 43 grains Cordite pushing a 270 grain projectile and '375 EX' is abbreviation of .375 Nitro Express Rimless, the British designation for the 9.5X57 cartridge.

You have several British proofs consistent with your rifle's time of manufacture and that the M1910 was popular with the British trade at that time.

Properly headstamped brass can be obtained from Quality Cartridge (Qual - Cart). Woodleigh still makes a .375, 270 grain round nosed projectile of the proper profile which, along with proper seating depth for precisely correct cartridge length, is vital for proper feeding through the Schoenauer rotary magazine. If you can find Hornady #3715, they also work very well.

Cases may also be formed from several 'donors' of the 8mm / .30-'06 'family'. I have had excellent results forming .35 Whelen brass through a two die RCBS set, then cut to length and stuffed with Hornady 3715 in the past. It is best to draw 'donor brass' straight prior to sizing, particularly with '_x57' brass as the donor shoulders will be lower and narrower than 9.5X57 which relies on its rather slight shoulder to headspace and to prevent stretching near the base.

Make your cartridges _exactly_ to dimensions shown in this Eley diagram and they will feed like melted butter on hot glass through your Mannlicher Schoenauer M1910:

Original DWM 531: