Looking good ... and an impressive bit of detective work too!
Thanks - I put myself in the "how would I do it if I had to" position with these historical conundrums.
The blanket dismissal that's been made of surplus Locusts being the M6 37mm donors depends on what "demilled" meant. In many cases, the breachblock was removed or chopped.
If that was the case, it would have been a simple case of going through the salvage yards of western desert vehicles and pulling the breach from an M3 Honey or two. Presto-chango, working 37mm main gun.