In a nutshell; this is the answer.
The FK1 and FK2 were full blown CPK models.
The BFK was a simplified model, originally intended to be sold on Amazon, so regular folks could get a taste of CPK’s offerings, and as such, the design omitted some of CPK’s standard features, so the BFK could be offered at a more modest price point.
After further consideration, Nate decided not to go the Amazon route, and just began offering the BFKs here.
The BFK and Kephart are in CPK’s Basic line, and are differentiated by a standard Carothers mark, vs the Carothers signature.
As numerous people asked if there would be a Signature version of the BFK, with all the other design elements included, Nate began offering those as the Signature BFK, aka the FK3.