Yes, it's an interesting piece. I don't know whether the SMLE over bore PPCo produced claw mounts pre-date the P'14 claw mounts made by the same company, or not. They were almost certainly made more or less contemporaneously.......I have a photo sent me many years ago by the late Bruce Gorton of a SMLE claw mount rear leg (admittedly for the Aldis rather than Model 1918 scope), which is clearly engraved 'Periscopic Prism Co London 1919'.
This Model 1918 scope appears to have rings designed to fit the SMLE claw mount bases. It would be unsurprising, maybe even be expected, for the authorities to hold comparative trials between equipments, & I wonder if this could have been such a trial or experimental piece.
All suggestions welcome!