Colorado, it looks like a spare receiver made up by BSA into a working rifle.
The barrel is a replacement, the R shows that a used '42 barrel was fitted later.
Other than the barrel and rearsight, the rest of the parts appear to be Britishicon.
The serial number is British, way to low a number for a 1916 Lithgowicon assembled rifle.
It may have had the original number scrubbed off, or have been a virgin replacement receiver, prior to 1925, the barrel number was the primary number and so if it was a part replacement, the receiver would have been stamped with the original BSA number, in this case I 8617.