There is an update to this story:
After some solid detective work on the part of the person who bought the pot from me, it turns out that it is NOT a Yamada family teapot.
It is made by another lesser known potter called Miura Jozan (三浦常山).
The interesting part is that this potter is from Sado Island. Apparently one of the first to make use of Mumyoi clay, made well known by Hojo tea!
On a side note, it turns out that MarshalN’s pot, which peaked my interest in the first place, is also not Yamada, but again the same Miura Jozan!