Wow, I'd have bet money that there's no way an old Honda 6-volt system could put out that voltage connected to a 6-volt battery. The only way I'd expect to see 10 volts at the battery with engine revved up would be to hook it to a 12-volt battery...
Kinda like the joke - "How do you make a small fortune in the commodities market? Start out with a large fortune"
Seriously, though, the rectifiers don't do any regulating at all, so about the only way you could get that voltage is if one leg of the bridge shorted across (and I've never heard of one failing this way) and you were reading the AC voltage direct from the alternator.
Keep us posted with what you find.
Kirk