1915 is wartime production, all the stops were pulled out for rifle production.
It is possible that a proofed barreled action or a complete rifle was set set aside as part of the production and quality control procedure, for rework to bring up to spec or to fix some problem.
Later, the rifle action or parts thereof were pulled from the stack and used in the assembly your rifle in 1915. The parts would be from a recycled rifle, which is quite a common practice at Enfield. The date was amongst the last markings to be applied
A proofed barreled action made in 1910 or earlier assembled into a rifle in 1915 is the most likely type of scenario.
An interesting example.