Perhaps a ball shattered. At any rate, at that age it's virtually certain to be the conventional cup-&-cone style of bottom bracket. Park Tool's site has some good info on this: link
There are several varieties of adjusting-cup fittings. Hopefully your left cup is the type that has holes in it, so you can just use one of these for the cup:
You'd need a lockring spanner of some kind too. I happen to like the Hozan forged dual-end model, but Park Tool makes some stamped ones that will get the job done too.
I have to agree that it seems less than advisable to stick with a 20-year-old bottom bracket if it's seen a full life's use already, but that's your call.