Allistair,

I can't tell you for sure but I had this problem with my '90 when I had it.
I suspect you are correct and it is a nut welded inside the tne box section
of the frame. I'm not sure how common this issue is but I suspect the darn
thing can get rusty and break off when you attempt to remove or replace the
bolt.

I didn't have the '90 long enough after this happened to fix the problem but
I had considered just having a new bolt welded to the underside of the box
frame there. The only problem I could think with that is that the tray
"clip" would no longer reach the tray - but with the bolt there it would be
sufficiently secure.

Biggest problem I had was that mine broke when I was removing the bolt - so
I had to cut the bolt to get the darn thing open.

Good luck.

--
Thanks,

Jeff
'85 GL (Gertie)
'90 Carat (Grover - the noble parts donor)
'78 Bus (Melissa) Patty's Bus