It does indeed look like a MOBAT proof shell. Very cool thing to be given
I don't mean to sound stupid, but what is a proof shell? Thanks.
A proof round is used to test the strength and hence safety of a weapon. It usually has a propellant charge 10-25% higher than the service charge. The projectile is either the normal weight or heavier depending on the requirements of the test and only needs to be a blunt representation.
Your example was made at Royal Ordnance Factory Birtley as part of Lot 2 and the "D?/L/8414/? is the design drawing number.
There are also proof rounds for projectiles, fuzes, primers and propellants but yours is for the weapon.
British Military Smallarms and Ammunition
Collector, Researcher and Pedant