Yes, that horizontal line is likely the seal where the back opens, up toward the ISO dial/latch.
See my PBase SQ-A page, toward the bottom. The shot labeled "Oh my Back!" is from a back with practically no seal left. It's the film coming off the supply spool that gets hit, not the film in place at the frame mask, so it's appearance can be variable depending on ambient light and how fast you wind through frames.
The good news is it fixable!
(Late catching up with this, having no power at home since Monday night.)