As we found out this is an earlier ZKK 600 and the later square type one will NOT fit. I sent a spare and it don't fit as I recall. My own ZKK 601 is of 94 vintage and has the later square type.
Steve you might want to check the proof mark stamp as it will give the year it was proofed which may help.
Hello Kevin, proof mark is 1977, still searching!
How goes it with you Kev?