Cobra top hinge plate bolt failure

I have an older (early 1992) Cobra with a fiberglass top. I haven't
spent the past 19 years on the road but the trailer doesn't live in a
hangar, either. I just checked visually and all four bolts are intact
and tight, the hinge plate is tight up against the aluminum cross
member, and there is no sign of movement or looseness or distortion
anywhere. I'll remove the bolts one by one to inspect but I'm
wondering, as at least one other person did, if the cause was simply a
bad batch of bolts, or perhaps improper tightening at the time of
manufacture. For sure, once there's looseness anywhere in the bolted
system, the likelihood of failure rises significantly.

Chip Bearden
ASW 24 "JB"
USA