Congratulations on your find. I agree with Davids comments. For whatever reason, most of these have ended up on this side of the pond. I have not seen or heard of them much in Britain or on the continent.
They come in two colors, a reddish brown and dull green. The makers mark is stamped next to the handle on the one side along with the Broad Arrow. They also originally had U-shaped felt glued in the top and bottom to keep the mags from rattling. The maker SPEEDWELL also dated theirs. I am aware of both 1942 and 1943 versions.
Congratulations on your recent find and I agree with Roger, these are not easy to find on my side of the Atlantic.
In my collection, I have two, both manufactured by SH Ltd, one green and one brown. I had been looking for one for a few years to add to my collection, and I was pleased as punch, when I was offered them by two MGB members, (not at the same time), so they were both shipped from the USA.
I read that in 1942, the British War department, approved contracts for the manufacture of metal boxes to hold 10 XX round box magazines. Examples of the boxes have been seen marked with SH Ltd., SWS Ltd. and Speedwell.
They were manufactured with the intent to simplify the logistics of delivery and five of the boxes fitted exactly inside a converted Thompson transit chest, with the inner dividers removed. So I need three more and a converted Thompson transit chest now.