The front two bolt mounting is the same but the rear end mounting is totally different. On the C15C/B44E plate the rear end is full width and is held to the footpeg/frame crosstube by two U bolts.
The B40 military plate is cut away at both rear corners to clear the footpeg bosses and the remaining middle rear end section is curved down at the end to simply hook over the centre stand spacer tube.
Yes, you could modify a C15/B44 plate to fit, which you would have to do, the B40 footpeg bosses being in the way of it. The B40 plate is also made from thicker gauge metal, if that matters.
Rupert.
Thought I would update my post to say I managed to source a sump/bash plate from, Burton Bike Bits (www.britishbikebits.com), Part: 82-8819 Engine Sump Guard - BSA B25/B44 / Triumph TR25W 1968-70
Fitted with minor hacksaw and filing modification to clear the right hand foot rest then a little gentle bending to match the curvature of the frame and it fits like a glove.
It has the curved down section at the middle rear end which hooks over the centre stand spacer tube.
Well pleased and it looks the business.