Funny story; in a mate's lockup there was a chap with a Plymouth Roadrunner, [edit, Superbird] Google it, the one with enormous spoiler. Not common, and even less so on this side of the world.
The horn had a decal informing the owner not to lose the adjustment screw as replacement would be difficult.
Yep. Japanese horn, so Metric.
Now I think of it, I wonder if it went "Meep Meep?"
2nd edit: yes!