I worked at Kollsman Instruments in the '80s. I wondered why the build looked familiar, especially the sealing.
I did not work on this line of instruments, but did work on the cockpit instruments They were hermetically sealed in many models.
I think it is hermetically sealed with metal tape and solder (SOHLder?).
After the unit is repaired and calibrated, we would send it to assembly girls who would use a huge soldering iron, old school huge copper head, but only 50 watts or so on a high mass head.
Maybe 3/8" wide, the tape, (think of thick alarm window adhesive metal tape) is wrapped around the bare metal, fluxed and soldered all the way around.
Then it is painted.
You do the reverse to get it off. scrape or clean the paint off. Find a huge old soldering iron, no flames or torches please, and start heating at the seam where the wrap of tape ends, you can pull it free bit by bit.
I'd freak if that thing came apart with 4 bolts hehe.