Dear All
Thank you for your replies and information shared, this was much helpful.
My rifle is similar to GG375 and Lonewulf's i.e. no Austrian marks (either proof or factory), British (London) proof marks only, and T in a circle underneath both action and barrel. One must accept - in this case, that it won't be possible to date it more precisely than 1908-1924, in the absence of any available archives from the London Proof House.
Best regards.
Louis