This man is a placeholder. There are two Juniata men named Martz (though I think originally Moritz): Amos and Solomon born in the 1815-1820 period. I do not think they were sons of either the original Jacob (U2) or his son Jacob (U2b) but I suspect, based strictly on geography and the county history statement about Amos that they were perhaps grandsons of the original Jacob. Otherwise, I have no better idea who they were.

Plus there was a Jacob E Martz of Sunbury who evidently had brothers named Solomon and Amos of Mifflin County.