I good start would be to check out the VJMC web site at www.vjmc.org. Joining would be nice, too! Honda never made a ´73 500 twin - it was only made for two years: 1975 and ´76. The ´75 only came in brown w/gold striping and a brown seat. They added a candy red in ´76. It is a fairly rare bike because it was priced so close to the 500 and 550 fours, which were MUCH smoother.
If you have a ´73, it´s either a 500/4 or a CB450.