The original horn in the M37 matched its original electrical system, 24v. If you have a converted M37 there really isn’t a 12v horn that directly replaces a failed horn. On my truck, there is a modern 12v horn concealed that works out, but I wanted to use the original part if possible. The original horn was corroded in the interior, so this ruled out using a 12/24v converter. Vintage Power Wagons sells the 12v HYF-370 horn, which does fit in the MX-4003-UT housing with a bit of fitting.
The screws holes holding the HYF-370 together need drilled out a little to fit, and the HYF-370 came with a gasket between the two halves so I used a small amount of permatex with the gasket to compensate for the slightly larger holes. The original horn housing is held together with a large hand turnable part (below) which the HYF-370 does not have a hole for, but it does have a bolt shaft in the center so a metal dowel works to hold the thing together. Shaping and tapping this was fairly straightforward, I used M8x1.25 and 10NF32 taps, but I am not sure how consistent this would be.
I put terminals on the wires, soldered the other end to the douglas connector, hooked it up and it works. Sounds appropriately truck like, even if it likely does not sound exactly like an original.
If you have an M37 with 12v, and your horn isn’t working, this is one way to get this working and still look mostly correct.