To follow up: I contacted S&W, describing the model and the problem. To my surprise, they sent a paid mailer, so off it went to them. I've just received it back, also on their nickel, saying the parts needed to fix it are no longer available. I'm disappointed, but I can't complain about how they approached this.

It looks as if I'm in the market for a nice, older 10. Sadly, a very nice 10-5 sold at the nearest store about the same time I shipped my 12-3 off to S&W. The search begins...