It can't be 375 2.5" as the Mannlicher Schoenauer will not feed rimmed rounds. I would suggest that it is chambered in 400/375 as I don't believe that H&H ever made a MS in 375 2.25" Rimless.
That rifle has been for sale on Guns International for ever, an interesting good looking rifle for sure but in a fairly used condition with plenty of marks on the stock. The price is obviously too high but I think that they are trying to capitalize on the H&H name.
If you do choose to buy, Bertram make the appropriate brass.

Waidmannsheil.