Good point. That'd be an issue with any verification system isn't it?
Edit to add: If you can confirm that all of the ids are unique and that the total number of ids are correct, that would go a long way towards verification, along with checking arbitrary receipts against the results, regardless if the ids are pre-printed or printed afterward.
I'd prefer not having any printers involved though. Pre-printed ballots would work just fine and there's one less piece of equipment involved during the actual vote.
Edit to add: If you can confirm that all of the ids are unique and that the total number of ids are correct, that would go a long way towards verification, along with checking arbitrary receipts against the results, regardless if the ids are pre-printed or printed afterward.
I'd prefer not having any printers involved though. Pre-printed ballots would work just fine and there's one less piece of equipment involved during the actual vote.