Just picked these up recently, the horns themselves look identical to the ones from my Challenger, Prestolite Hi and Lo, but the bracket has me stumped as to what they came off of originally. If I have the date code right, 8C they were made in August of 1970.

I have never seen that bracket before. Interesting.

I agree I have out seen them either.

Could they be a different brand? Prestolite wasn't just a mopar only part was it?

That seems like a possibility, I noticed my Challenger's horns are stamped in the week - year format, 47 9.

just recently, a good buddy and i were at a recycle yard looking for horns for his 54 chevy.
that exact mounting bracket and horn assembly was what he picked up from a [somewhat] newer 2000ish gm vehicle like a midsize, or what small car passes for that these days.
they are located behind lots of plastic panels around the grille area on the driver's side of the car.