How do I remember/mark/flag/access the original faces of a mesh?

Warning - I’m a bit of a scripting noob. I tried to search for a solution but came up with nothing.

Is it possible to flag or mark the original faces on a mesh so that after you have added more faces to it, your script can differentiate between the original faces and the ones you just added?

Or is there a better/built in way that I haven’t found?

It depends a bit…

If you work with the bmesh module, you may hold a list of face references, or tag the relevant faces (bm.faces[#].tag = True).

Beware however, that the tagging state is copied over to new faces on duplication or similar operations (properties are inherited). Splitting a face into multiple faces will make all new faces have the properties of the original one, but if you extrude a face, the newly added faces around the translated one won’t.