Did you ever figure out what the spoke size was? I'm in need of a single replacement spoke and have looked at buchanon but I'm not sure what size I need. I have a 1980xs400 16" rear drum brake.
I also see some kits and partial spoke kits around for a 16" xs650 rear wheel and I'm wondering if those would fit as well. $35 for a partial kit seems better than $75 for all of them...especially when I only need one.