Estonia had large quantities of British military aid after WWI, including Lewis guns and Pattern 14 rifles. There are plenty of pictures of Estonian troops armed with P.14s. I included Finland because the style of marking looked vaguely Finnish to me. Whilst I have no confirmation that Finland had a...

Just to be difficult, I would question whether the stamp is even British.

P.14s arrived a bit late in the day for an old county Militia stamp and I would expect a date of issue to be present on a British disc.

Perhaps Finnish or Estonian?

Regards

TonyE