I have a homemade 4x5 back for my 8x10. I have a Deardorff 4x5 Special that comes with a 5x7 back. When I had an Elwood 5x7 enlarger I shot a lot of 5x7 as I had a 5x7 Kodak 2d that I bought from the Fort Worth Police Dept. They used it for mug shots back before that Polaroid thing. I have use of a Deardorff V8 that has a 4x5 and a 5x7 back. I think the reducing backs are pretty common as least around here.