I would agree with what has been said already in this thread... if that K-22 is actually a 4 screw and it is in 90% condition as described, then $600 is a fair price to you and him. I, like smithrjd, have a 1948 vintage K-22, in my case an 80-85% finish one bought at auction about 5-6 years ago at what I thought was a screaming deal for $425. I'd buy the gun in question in a minute if I were in the market for a K-22.

BTW, the 4 screw configuration is probably something of a bonus... it's the variation that was made for the shortest time. Not a huge premium, probably, but worthy of note.

Regards,
Froggie