I'm glad Kuduae weighed in, as he has far more depth and breadth of knowledge on these matters than I.

As he indicated, some military MS (Y1903) made their way to gunmakers (some say as bare actions, others that Steyr sold only complete weapons) who altered and customized them. Included were 'bespoke' makers such as Gibbs, Westley Richards, etcetera. These arms lack Austrian proofs.

Usually, my first thought of a non military 6.5X54 MS without Austrian proofs would be of a 'Sporterized' surplus arm or a 'Greek' which was reconfigured by a custom gunmaker when new.

The OP's receiver, however, is in (factory?) take down configuration and has the bolt hold down spring as detailed by Kuduae.

Most or all of the Steyr factory built Take Down rifles of which I have seen descriptions and / or images have borne both Austrian and British proofs as does my own M1910.

It seems that with the Mannlicher Schoenauer there are few 'hard rules', and that anomalies exist.