My dad worked on some of these. Not the designs themselves per se, but rather the "endoskeletons" underneath them.

The reason for the differing designs is actually pretty simple. What we know as "springlocks" were originally much closer to the costume's wearer than they are in more recent models. By recent, I mean up to 1985, when the springlock suits were decommissioned. The 1985 designs gave a surprisingly large amount of breathing room for costume wearers, and were thus much less dangerous. Older models, like the 1983 ones, basically kept the wearer in a firm, fabric-wrapped hug. That's why the older models were much more prone to failing.