Further to my last. I find that Slab Dem CE/TNT 1 lb issue started about 1943 and would have been the slab of choice to those at the sharp end i.e. Royal Engineers, SOE and other clandestine organisations. The Wet GC slab was nowhere near as powerful as the CE/TNT slab, furthermore, it was liable to deterioration over time, so was not that reliable.
I looked at a 1960 publication and amazingly it was still there, although deleted, but left in the book.
You can clearly see your lid. I thought that is what it was because of its size and what appears to be a rubber seal around the edge.
Depot demolition grounds were littered with white cellulose block pieces in the late 1950s.