My reaction exactly. That coupled with the original caliber of 8X57JS makes me think that it was sold during the time when W-R were simply using standard Mauser barreled actions and stocking them in their own distinctive style as well as adding their own typical sights. The low serial number would seem to bear this out, placing it in the 1912 range, while Rigby were still the sole agent for Mauser rifles in Great Britain, and shortly after W-R introduced the .318 in 1910.

KWJohnston: As best I can remember, the buttplate is plain steel.

Incidentally, before being rebored, the caliber was stamped on the side "7.9 mm". I simply had the "7." removed from the marking, leaving an authentic looking 9 mm in its place.