I have an old M570 Trackball Mouse that no longer works. I'm just getting into repair and I thought it would be good for practice. However I'm having a lot of trouble getting it open.

The shell has four Phillips screws on the bottom. I removed the battery and pushed the ball out of the case. I removed those four screws and set them aside. There are no other screws visible.

I can get a Jimmy into the crack between the upper and lower parts of the case, but I can't pry it open. There seems to be some additional fastener that I can't see.

I don't want to break the case by brute force. Is there something other than those four screws that I need to remove?