My understanding is that on some of the no dash 686's the doohickey in the recoi shield where the firing pin comes through was too small and could be pushed back in it's hole from recoil and the fix was to install a larger one. But apparently, not all 686 no dashes had this issue. I have read that if the gun has fired fifty or so .357 rounds and the doohickey looks in it's proper place, that all should be fine. So I'd just keep a real close eye on that piece as you shoot the gun... and if it looks like it's pushing back, send it to S&W for the fix.
Nice gun and excellent find... someday I hope to find one of those myself.