I just check an orginal wiring harness from a 1964 Beaumont that I know has not been changed
as my wife had bought it new and had it when we married in 1970.
The horn relay says "Delco Remy made in USA"
'64 Parisienne CS "barn find" - last on the road in '86 ... Owner Protection Plan booklet, original paint, original near-mint aqua interior, original aqua GM floor mats, original 283, factory posi, and original rust.
I have done some checking at car shows lately on GM cars and
it seems that most have Made in US horn relays but have come
across some that look stock (not after market) and have nothing on them.
Just a thought. At least it won't say USA.