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.
The ones I have (nearly run out of) are semi print thru, like the ones found at this link: http://www.twincheckphotography.com/...winchecks.html
Admittedly its only the landing page for this guys site, but it illustrates the point!!
Here in the UK, a couple of places sell them and they're quite cheap. The adhesive is quite good and they don't come off during processing. The hardest bit about using them is attaching one to the end of the roll of film in the dark!!!!
Thanks for all the replies.
I went after some twinchecks in Brazil. Haven't had any luck finding them, but I may have found a way to get them from China.
if your still looking for twin checks i found some here