I am currently looking for the correct Phototransistor to build the Shutter Tester and in the meantime I thought about an efficient process to enable the testing of the Shutter at all Speeds at No cost !
Here the trick, it involves close contact between a Digital Nikon and the M2
1) Remove the back from your M Camera.
2) Procure a reliable Digital Camera with possible M mode (manual) on which you can select the Bulb mode.
3) Start with the 1s Test. Position the Leica M on B mode and keep the shutter open.
4) Position the shutter speed on the Digital Camera to 1s.
5) Locate the Digital camera right behind the Leica M with its removed back.
6) Fire the 1s on the Digital Camera.
7) Note the obtained picture number on the Digital.
8) Switch the Leica M speed to from B to 1s.
9) Switch the Digital Camera from 1s to B.
10) Trigger the Digital camera shutter to B and keep the shutter open.
11) Trigger the Leica M on the 1s.
12) Release the digital shutter button immediatly after step 11.
13) Compare picture from step 7 to the one from step 12; they should have the same exposure rendition.
14) Repeat steps 3 to 13 for all speeds
15) Open a cold Beer and enjoy !
Note : to avoid parasite light to disturb the test, cover gap between the Digital Camera and the Leica M with a black Tshirt or a black cloth.
With this test my M2 reveales no differences up to 1/500s. Only at 1/1000s there is a slight difference in tone most probably due to the lack of real 1/1000s at this set point. Quite normal for a Leica M and actually OK for practical use.
Here are the pictures related to this test: