The broken link bar rivet could be a total fluke.
Maybe inspection of the others will shed some light on it.
The thing about the old MP cams is, they were designed to provide a fairly high level of reliability, with modest opening and closing rates, so they could be run with factory rocker gear.
They also had pretty quiet operation...... another tell tale that things are operating in a fairly smooth manner.
I’m sure you’ll get it sorted out.
No idea on how many miles those solid rollers were in service?