Ok, thanks for that. Disregard what I mentioned earlier.... I was referring to the battery compartments, which are accessed from the outside, on each side of the tub.
The compartment you have pictured was just used for storing detachment stores, radio accessories or tool kits. That frame isn’t the standard for the radios... I think I have an idea what it was for, but I’ll check that out a bit further first and get back to you.
Cheers.