It might be handy to make up your own copyright and artwork to suit the space at the end of a 30,5cm sheet. Print it out on A4 on a laser printer etc, together with a few sets of numbers 0-9 (and/or letters). Shoot a few shots. Project-print on to sheet film backwards, then contact it for reversal.

Oops, I might be getting carried away with enthusiasm there, it could be much simpler to use Letraset (rub-down transfers) on some pieces of acetate, as that could give you individual numbers (say a 5mm high, or so) and letters/words which you can sandwich with the negs when you contact them?

It should also be possible to get a local small-business printer to print clear self-adhesive labels with changing text (incrementing numbers say), so that would be another option. Possibly not so archival though and, potentially, labels can fall off too easily.