I've only noticed the numbers on the backing paper now after fishing it out of the waste paper bin. They seems to be for 6x45, 6x6 and 6x9 right. What about 6x12 and 6x17? I guess you just count up every so many numbers right? Coming across these backs has made me even more interested in the tech v now. I never knew I'd be able to experiment so much with it.