Welcome Chris, and we'd very much like to hear more about your family history and Hunter Arms.

To clarify, the barrel flats and action flats carry the same SN, and the action flats have these patent dates?



The most likely explanation is that the FE was separated from the gun, and the current FE is a replacement.
It is possible that a Research Letter might document the FE replacement if done at Hunter Arms, but if so, the FE iron should have been marked with the original SN
https://lcsca.clubexpress.com/content.aspx?page_id=22&club_id=43784&module_id=245742