By K0 I guess we are referring to die cast engine CB750's?
As pointed out by KP, HM300's were fitted from "late 1969" to die cast engine CB750's. My un-molested 1008054 (9/69 build date) still has its factory fitted No-no exhausts. So HM300's first appeared somewhere between 1008054 and 1011970 then?
I also had a highly original K1 (VIN 1067xxx I recall) which came with a number 4 exhaust without brake pedal relief.
Chris R.