Welcome to the A12 box collectors network - membership 4 as far as I know!! In fact there must be more as a a couple more boxes sold sold through Facebook last year that were too far away for me.
There are a couple of other posts in this forum on these boxes including one with markings for Livens Projector charges. From my collection.
All these boxes were made by Electrolux, never seen any by another manufacturer. Dates 1939-43 with use by Auxiliary Units and possibly SOE. Later still the box was made deeper and the A12 removed though the ELX and date remained. These never had the round discs which clipped the charges in place. The large size of these made them popular for soldiers shipping belongings.
Your 1939 box should be deep bronze green with rubber handles.
If it's the one ex-eBay A12 from Taunton way, then it looked in fairly good nick with original paint and surface rust. The four digit number on the front I've seen on one other box. No idea what this is for. Aux Units would have just under 10,000 packs in these boxes so I like to think it might be that, explaining the lack of other markings. That may be wishful thinking though!
will