This finding would suggest that the above proposed sheet layout is not correct, since it is likely that the *Z/Z6400000 note would have come from the same sheet as the other 6 in the NCC, and the other 10 known notes in the same range.
Given that sheet numbering layouts for these specially produced sheets didn't always follow the established patterns I can see one possible layout as:
6380500 6381000 6381500 6382000 6382500
6383000 6383500 6384000 6384500 6385000
6385500 6386000 6386500 6387000 6387500
6388000 6388500 6389000 6389500 6390000
6390500 6391000 6391500 6392000 6392500
6393000 6393500 6394000 6394500 6395000
6395500 6396000 6396500 6397000 6397500
6398000 6398500 6399000 6399500 6400000
Another possiblity is that this part of the *Z/Z run was skip numbered by 1000 rather than 500. This may be a possibility as is seems that most of the notes from this sheet were preserved, and the preponderance of 000 notes and the lack of 500 notes is quite noticeable, although in other areas of the *Z/Z run there are 500, 499 and 999 notes.