The plastic carriers required manually inserting the IC in the carrier and inserting a plastic "window" which had incredibly small plastic clips that would lock the IC in the carrier. This was a manual process by humans, thus prone to error (windows not fully snapped into place). The error rate wasn't too bad, usually less than 1% failure. Certain vibrating bowl handlers with a vertical ramp (towards the ceiling) and ~180° radius back down toward the DUT interface were the most problematic. I think the mfg floor phased out those problematic handlers, eventually the flat pack package became somewhat obsolete with the advent of smaller, lighter surface mount packaging options.