Thanks for all of the quick replies.

My own instinct is that it was a manufacturing defect that probably just needed a little push to emerge. Maybe one head was already cracked, lasted for a while and when it went, the change in tension set more up to go. It might be interesting take a pair of pliers and see how many more heads are ready to pop off.

Used bikes are a gamble and you're right, without knowing the history, you can only look at the evidence and use your experience, which I don't have much of, in this area.

I did several searches on the combined terms; nashbar, maruishi, 1985, spokes, "broken spokes", defective, etc. and came up with nothing (now the search will bring you here). I've made a note of the problem on my page about this bike, with a link to this thread. It may save somebody some trouble or provide a bargaining chip when buying a used '85 Nashbar bike. Unfortunately, these spokes have no distinctive way of identifying them other than looking very generic.

I had the bike back on the road a few days ago, for a 20 mile ride with the respoked, properly tensioned rear wheel and there have been no problems with it. It's a nice stable ride but for my own peace of mind, I guess I'd better rebuild the front wheel as well. I'm hoping to do some more longer rides with this bike and I don't want to be preoccupied with potential problems when I should be just enjoying the ride.