[Lynn Eades]

Rob, I believe what you say is correct, however the MkI* U.Cs of the 41-42 era were all fitted out to carry a Boys (in the right rear compartment) The fitting out of carriers to suit different uses in the platoon came later, or have I got that wrong?
With the pre universal carriers, the large locker was for Bren (gun) spares, according to the early stowage plans and this leads me to believe (as you say) the carriers were issued with more than one Brengun.
Later in the war the Boys was probably scaled back as armour grew thicker in the same way as the USA's 37mm anti tank gun became obsolete.
Tom, the blanking board was originally only 3 ply (one thick ply in the middle) about 9 to 10mm thick.
If i remember correctly the ship lap boards were about 32mm thick and something like 115mm wide.