More trivia.....
Early CB250 twins used a horn with a "gray" body, but different stay.
The horn from a 350/4 honda is effectively a left hand sandcast horn (black body, chrome cover with drain hole at the bottom and cable exit at upper left) but again with a different stay - replace the 350 stay with a 750 one to convert.
These at least provide ways of re-creating a left side horn if missing.
As per Steve's previous message, all the left hand horn sandcasts I have seen have black body / black sheath. This includes my two bikes (#'s 302 and 1120), and the NOS left side horn I have.
Anything else is either one of those mythically very rare factory fitted ones, or perhaps not original to the bike.
Cheers - Chris R