Thanks for the response and a lot of good info provided.
My M1910 is a Half-stock ~20" barrel so I would call it a carbine. All the proofs are similar to as you explain. The London are interesting of course as it specifically says the 43 grain cordite and 270 grain bullet and .375 EX. The stock has the serial number of the action under the front receiver ring. There is a +2 and -2 similar to your "15".
With your gun being involved in the known Ceylon trade it certain adds credence to the thoughts that the impressed stock toeline numbers had something to do with guns sent to India/Ceylon.

If anyone else has information they could led please comment. Thanks to all