Honda never sold blank keys as far as I know, you bought the key precut using the key code number. So if all you have is a copy with no number... ?
Some models have the number on the ignition or seat lock but not all.
Hondakeys.com is the place to try, or maybe an long time Honda dealer near you has a dusty box of old keys somewhere though. Honda dropped a lot of local dealers a few years back to boost their Powersport stores though...
There were actually very few keys for these bikes... I've heard the early 750s had 7 different keys. If you can find that dusty box of keys and start trying them you may get lucky.