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!!!!