I have a couple 8x10 holders loaded with film with a single, narrow notch. I cant tell if its a V or a U, but I can tell its not very wide. Its narrower than the U notches in FP4, for example. I also know it has to be either Ilford HP5 or FP4, or its old Arista/Ilford equivalents because those are the only films I've ever used! But it doesnt match up to any of those. I know a lot of European fims have a single notch but I've never bought any of them. It pretty much HAS to be Ilford/Arista 400 or 125, no more than 10 years old