Success !! Thanks to Liz's Antique Hardware, was able to put together a group of matching period-correct door knobs for the entire house.
The "Plymouth" knob is actually still in production but the older ones look a little different. Brian at Liz's was extremely helpful, digging through box after box of hardware to come up with a mix of new old stock and vintage knobs.
Restoration is moving along very quickly, won't be long now, stay posted.