I'm pretty sure the SQ-B will take any of the SQ backs (6x6, 6x4.5 and 35mm). Some of the functionality of the "i" backs are not available on the SQ-B (electrical links to the metered prism, ISO setting etc), but they still fit and work. The ETR backs are only for the smaller rectangular format ETRS/ETRSi cameras (6x4.5 and 35mm) so you'll need to avoid any of those. There's also the larger GS-1 camera with 6x7 backs, but it doesn't look like they've got any of those!