Satin Jacks are superb knives. They're not actually rare; they're one of the most produced Busse knives.
The 6" PBF is rather similar to a SJ. If you were to start with an SJ, make the point pointier (but also thicker toward the tip), make the body of the blade sturdier and a hair wider, make the edge profile more acute and sharper, make the guard more protective, make the grip more secure, and make the balance faster, you'd end up with a 6" PBF. When you see them side by side, it appears that the 6" PBF started from an SJ pattern. The 6" PBF is like an E series Satin Jack on steroids.