[cmperry4]

I started to remove the old pressboard liner panels in the HUP today. The first one I unscrewed was the top piece on the inside of the spare tire recess. When I pulled if off there was a piece of wood lath and a metal piece resting in the top of the "X" of the carrier recess - lo and behold, it was the cover for the "cupholder" - the mystery opening on the top of the doghouse. What's more, it was painted in the original olive drab, not the oobleck green, which suggests it had been sitting there since the '50s, when I figure the RCAF did that interior paint job.
I'm guessing someone was working on the engine, had the doghouse in the back, took the cover off for some reason, rested it on the window ledge, then maybe vibration caused it to fall off an slip behind the pressboard. It was either looked for, presumed missing, or not missed and back went the doghouse sans cover. No theories about that piece of lath though.
A surprising and pleasing find, to say the least.

This shows it, where I found it, and where it belongs: