...just went out and looked. it kinda seems the 400F switch is the same. there's a bit of a cut-out in the top clamp for the switch tube, then it's mounted up into the bottom of the clamp with two bolts. i'm not sure that it has the flange on either side like the one on the old ebay auction though? i'm happy to send it/ride it down if you think there's a chance it'll fit, or if you feel you can adapt it. or let me know if you think it's worth taking measurements and pics. i have two spare, so you're welcome to one.
the 400f switch i don't think will work...a couple of reasons: 1. i don't think it mounts to the bottom of the top triple tree like the 500T's. (500T's other 2 small threaded holes are for the instrument panel to screw into) 2. the 500T's also got a retaining clip that goes around the base of the switch. if it looks like this http://cgi.ebay.com/HONDA-CB400-CB9...ryZ10460QQrdZ1QQssPageNameZWD1VQQcmdZViewItem
it won't work. b/c i already tried one just like this...i appreciate u looking though.