A common problem to period belt buckles of this vintage, 73 74 is the buckles that are "Exploding Belt Buckle". (A term coined by the classic car crowd) The guts simply seem to fly apart at some time. One of the nipples that hold the plastic together breaks off and no longer hold the buckle together. This must have happened to the passenger side because that side had one of the back coach belts installed with the original belt nowhere to be found. I really like the GM Logo so a replacement solution should be similar. There are parts available though. Here is one source for purchasing expoding buckle repair parts. But a search for exploding seatbelts will give you more.
No portion of this page, text, images or code, may be copied, reproduced,
published or distributed in any medium without the expressed written permission
of the copyright holder.