Hey, guess I'll just go and eat worms.
On rechecking the old RCA databooks, where secondary breakdown parameters are actually specified, the correct notation for secondary breakdown current is I sub S/b. Doesn't Google very well.
It once had the same ~status that Ias had in specifying mosfets, though the breakdown phenomena differs.
You can display a V/I plot. But modeling secondary breakdown is still a bit of a pain, based on what I've read.
It has to be derated linearly with temperature, as with most power-related characteristics. The Rthjc of the larger Motorola plastic package is around 4degC/watt.
Another thing you couldn't sell; a supply that needed a fuse replacement, every time the outputs were shorted.
Even in '68.
RL