He doesnt state where or what and as its 2019 who knows what was going on unless he tells us (in general terms).
It could have been involved in a tussle in the cookhouse at the hot plate,a fight over the remote in the TV room, or one of the storemen could have issued it to himself.
Ive met a lot of bluffers lately, I always ask for their service number,mostly I get I cant remember ive got PTSD or some other drivel.
I dont know one genuine squaddie who's forgotten his number whatever the circumstances.