Unit Marking?

I have looked up and down Ian Skennerton's book on The Lee-Enfield and have not located what this marking on the butt plate of my BSA 1918 No. 1 Mk. 3*. I assumed it could be a unit marking, but I cannot find any information on it at the moment. If anybody here could help I would appreciate it.


Edit: Is M/216 a code for the manufacture. That is at least what I have found after digging a little deeper. Specifically Sanbra Ltd, Brimingham?

Manufacturer's code seems correct.