I have obtained an older M3 (1957) and it has a light meter attached to the top.
I can NOT seem to be able to remove this meter. I do NOT have a manual for this camera. So I was hoping that somebody here might be able to give me a clue as to what combination of levers allows the meter to be removed.
Looking down at the camera so the lens in pointing away from you, as if you were going to use, turn the knurled exposure meter knob counter clock wise still it stops. You then should be able to grasp both sides of this knurled knob and lift up. This will pull the connecting pins out of the socket on the camera so you should then be able to slip the meter out of the shoe. Be careful not to scratch the top of the camera if you can.
If you intend to use the meter you probably need PVC tape on the top plate of the camera otherwise you will probably get (more) rub marks.
When you lift the knob you have to turn it some more (same sense) to lock the knob in the raised position, so thet the meter can be slid back from the camera without marring the shutter speed knob.