Leicameter MR-4
an accurate meter for the meterless M
Coupled light meter, the Leica Meter Model MR - 4 (black version shown mounted on an M2), similar to the MR meter with the major change being the relocation of the metering button to the top of the meter so it wouldn't foul on modern Leica cameras with angled rewind cranks such as the M4.
The MR 4 is a reflected light meter, the meter is mechanically coupled to the camera's shutter speed dial.
A neat feature of these meters is the ability to select the 90mm framelines in the camera, the area within the 90mm framelines is the metered area.
The meter originally operated off mercury batteries, however they can be modified to accept non mercury batteries. Most professional Leica service people can offer this change.
The Leica Meter wasn't actually manufactured by the Leica company, but by the Metrawatt company in Nurnberg, Germany. Metrawatt had made several previous light meters for Leitz Cameras, but the MR4 was the most advanced and useful.