After tons of research I think this is correct, Let me know if it needs to be corrected:
68-69 CB350K0 was released on 2/29/68 and was made up to the K1 release REAR HINGE SEAT
69 CB350K1 was released on 05/01/69 and was made up to the K2 release also REAR HINGE SEAT
1970 CB350K2 was release on 02/10/70 and was made up to the K3 release also REAR HINGE SEAT
71 CB350K3 was release on 02/08/71 and was made up to the K4 release NEW SIDE HINGE SEAT
72-73 CB350K4 was released on 02/01/72 SIDE HINGE but now the latch uses a key.
My (what I thought to be 1971) has a plunger that goes into a locking mechanism, when you turn the key the seat comes up because it has a big spring in the middle section under the seat. So now I'm thinking my 71 might really be a 72. Hmmm.....