The marvellous example in my custody simply says
256
below the Nitro Proved mark
and amongst the British proof marks on the barrel.
The rifle is an original W.J.Jeffery & Co Steyr Mannlicher sporter marked STEYR 1900 on the action.
Jeffery's full London address is engraved on the barrel between the express sight block and the receiver.

'Spose I should get some photos organized while sitting around in self-imposed corona lockdown.