Why 45RPM's reissues numbered and not numbered?

Rushton is spot on.

The first 1000 are numbered. After that, not numbered through the rest of the production run. There's no difference in quality. As with any LP production, the earlier the pressing made from a given stamper, and the earlier the stamper, the better the result. But the first LP pressed from the 2nd stamper may well sound better than the 500th LP pressed from the first stamper. I've not been able to tell any difference in quality among the various LPs from this series that I've acquired based on having or not having a number.