I use a 460MG head on my 055. It has a QR plate similar to yours. When I mounted my Bronica SQAm on it last week, it was quite loose and the safety lock didn't seem to work. The camera fell off at one point, but I managed to catch it. The solution was to mount the camera with plate attached to the head, then push the big locking lever tight. The camera should then be secure, and the small brass lock can be engaged. I had wrongly assumed that the plate engage securely simply by attaching it to the head. Alex