You know, as I ponder over this I think you may be right. I don't' recall doing it but in all likelihood I may have gotten aggravated at the nylon bolt getting stripped and got a metal one. The only thing is I heard a snap like sound when I took the enlarger off the table to pack it away ans shoved it past the door and into the storage area. It could have been I took that nylon bolt and nut out, tucked it behind the baseboard where you couldn't see it from the front and forgot about it.
Years later after putting the enlarger away, it could have fallen off on the concrete floor of the storage room and somehow got underneath the baseboard by it's edge and when I moved the enlarger over it snapped giving that sound. Pretty good logic and definitely not impossible. If you look at that original bolt/nut you can see where it was stripped.