Interestingly (to me), the eBay NOS horn (which BTW is the same diameter as my OE horn so parts swapping *should* work) came with adjustment instructions. Basically there are no adjustments. You're not supposed to play with the big adjustment screw and nut that were adjusted at the Jubilee (TM brand) factory but you can adjust the little screw as the points wear with time. OKAY. That might explain my issue with my original horn. It might be so far out of adjustment that it's FKd.
Here is the new horn, soon to be sacrificed for its parts and the instructions: