I also had seatbelts in my '53 M38A1. Same deal, military type, although mine had a different type of buckle. From what I read (sorry, can't source it currently), seatbelts were added by the motorpool to some jeeps as a way to make them "safer". However, what I have does not fit what I would consider to be the "safe" category. While one end was bolted to the frame via the M31C mounting plate, the other was simply bolted the sheetmetal of the fenderwell with two flat washers on either side. Stress cracks had already begun to develop in the sheetmetal. I opted to retain the seatbelts and fix them up a bit just because they were there and the holes had already been drilled.
My jeep came from a chemical weapons depot. It also had red reflective tape added to the rear corners. Kept that, too.
1953 M38A1 MD54216; 20988792; 40-I-160I B-35
1954 M170 MD-A10513; USMC
1964 M416 1/4 Ton Trailer