I think Flea uses their own capsule now, but don't quote me.

I wish someone would do an acoustic guitar or vocal comparison between the mics you mentioned. I have literally never hear the Upton C12, but it seems to be pretty accurate, plus I love their 251.

I feel like if you want one as close as possible to original youd have to get something custom. Closest to vintage would probably be a Flea mic body/hardware, Campbell capsule, NOS tube, Haufe or Moby transformer. I wonder if there are boutique builder that put together mics like this.