Andrew, I don't think that's camo as it's on the floor as well. I'd suspect mould or watermarking on the photograph...really serious mould that I wouldn't want to breath in if it's eaten into the glass negative...
My Norton has Light Green No.5 in a diagonal pattern as the remains of the finish were still visible and it didn't see service after 1940.
The contrast between KG 3 and LG 5 is not that great. The disruptive pattern is more distinctive using Dark Green No. 4, as per the tank on the French M20 posted yesterday.