It looks very much to me like you pulled the spring back away from the camera body when pulling or inserting the darkslide. Pinch the back to the camera body with one hand when pulling or inserting the darkslide.
Then again, since you did some repairs, maybe the spring back and/or the filmholder are not seating properly. Do check that as well before going photographing again.
I, too, am puzzled by the "rock," or whatever it is bottom center. It appears to be some kind of defect, but I'm not sure. Is there damage to the negative?
Best,
Doremus