If you take a right hand horn, remove and reverse the mounting bracket, it can be fitted onto the left hand side. BUT, the chrome cover is also "handed", such that the wires would exit to the outside of the bike in the case of a right hand horn mounted on the left.
I believe though that the chrome cover from either a CB250 / 350 twin, or else a 350/4 (can't remember which right now) is the same as the "left hand" CB750 one - I have by mixing parts in the past created a correct left hand horn in this way.
I am not going to mention the subject of grey bodies, or grey wiring sheaths at this time (oops, I already have I think!), as these only seem to have been fitted to a very limited number of mostly very early serial numbered bikes - and we don't have any absolute data from which to pin this one down.
Hopefully the two attached oictures will show you what an original left hand horn looks like, and where the wiring exit is on the cover.
Chris R.