OK. I missed that the half nuts came from a different seller than the lead screw.
Yes, on there having been two versions of the 618 half nuts. I never knew exactly what changed because we don't have the drawings, just that something had changed. If you look at the 1945 and earlier parts manuals that we have, you will see that they say part number M6-12 for the Half Nuts, M6-13 for the guide and M6-12X for the complete assembly. The 1950 and later says M6-12A, M6-13A and the complete assembly is M6-12AX. The difference appears to be the width of the half nuts and the width of the slot in the guide. So those changed sometime between 1945 and 1950. If I remember, next time that I talk to Tom at Clausing, I'll ask him whether the drawing gives the serial number at which the change happened.