Petri FT 35mm SLR circa early 1970s

Petri ft  Slr film camera dated to early 1970s

Petri FT I have had this camera for over 20 years it still works but for some reason the battery cover is stuck I cant unscrew it ,the camera is very well made and it comes with a petri lens 1:1.8 plus a 2 times doubler ,,the lens mount is a petri breech loader unique to petri I suppose it couild be used with the sunny 16 rule ,the company started in 1907 but the camera dept folded in 1977,the shutter  and timer still work and the lens is clear.I have another FT model it has a problem the frame advance is stuck any helpful comments welcome