I picked up a X-370 for my mom and re-fomed it last week. Yesterday I got a lens from her in the mail to run some film through to see if I was successful. It was dark, so I broke out my modern flash (Nikon SB-80 ca. 2000). It fires the flash, and the shutter actuates, but the mirror doesn't return unless you turn off the camera!
Just to be sure, I shot some hand held at 1/8s, and everything worked perfectly fine. The flash works fine on my Nikons and using a PC cord to my Mamiya.
I'm hoping it's just an issue with new-fangled flashes, not the camera. This body will replace her SRT-101, which she's got a vintage Vivitar 285 for.