I remember seeing what you describe, I think. It was a block of metal that fit in under the open frizzen, and wedged down over the pan, with a nipple screwed into it. The flint was removed, and a small square hammer face replaced it. The pan was primed just like a flinter, but was ignited by a cap.
I think, whoever was making them, quit after about the third running gun battle erupted at a rendezvous, when somebody shot the flint trail, and then put one of these contraptions on, and shot the percussion trail with the same gun.
Hungry Horse