I thought it sounded a bit odd. After all there's no shortage of 6x9's about and not many of them have anything more than a ruby glass as a frame counter. So if you leave left out the 6x9 frame numbers you'd be turning down money.
I was out with my Kodak No 1 Pocket camera today loaded with some slightly out of date Illford delta 100 to get some pictures in the snow. But it would be nice to do some transparencies in 6x9 just to view on a lightbox. So I was a bit concerned that I might not be able to get any 120 Fuji slide film with the correct numbering.