Mark - the gun from the Florida museum looks to be a very old (period) restock of some very nice, relatively early (ca: 1790s-1810) iron hardware. I don't think it has a step. The original gun it came off of might have, but hard to say - based on the proportions of the guard, I don't think so - but who knows. It's real interesting since the patchbox looks to have ties to Jacob Young, the guard looks like Joseph Bogle, and the buttplate looks like neither of them. As it is stocked now, it has a half round cheekpiece and slight "fishbelly" profile to the toeline which were popular in the percussion era.
Interestingly, the iron mounted Jacob Young rifle that turned up a few years back, which looks later (1820s), than the two brass pieces (1800-1815) has a sliding wooden patchbox....so Jacob varied his work.
The captured lid box was in use by a lot of gunmakers by the early 1800s, from Virginia southwest all the way into Tennessee, up into central Kentucky, and over in the western piedmont of North Carolina. There are infinite variations on it - there are even east Tennessee rifles that combine its use with the classic east Tennessee "banana" box. So there is a lot of room for running with an idea inspired by the work of Mr. Young - one of the great, relatively unsung gunsmiths of the early transmountain west.
Guy