I engraved a few more nut, but have quite a few to go and am really happy with the outcome. One of the advantages of engraving the nuts, is that the holder can be reassigned to another number by changing the nut.
I still need to work out what to do with the holders that don't have nuts. I may use CNC Wrapper and engrave the number on the circumference of the holder, but for the moment, I'll keep using the stickers on them.
Cheers,
Peter