
Early M2s have no plastic plug at the end of the rangefinder to block light. Light was blocked with a light shield, which looks like this, having a cover on the end of the light shield to keep stray light out of the rangefinder.

Like this.

Later M2s had a plastic plug which was inserted into the end of the rangefinder to block light. The light shield for these was missing the metal piece on the end since it was no longer needed.

Early M2 light shield on the bottom, later light shield on the top, with no end flap.

Later M2s came with a plastic plug at the end of the rangefinder (top). On earlier M2s, this was missing, and stray light was handled, apparently not always that well, by the extra flap on the end of the light shield.

Later M2 design.
If someone does a CLA on an early M2, and replaces the early M2 light shield with a light shield from a later M2, (don't laugh, it happened to me) and does nothing else, you will get a light leak.
Alternatively, it appears that the early M2 light shield, by itself, as used in the original design, without the plastic plug, may have had occasional light leak issues, which would explain the design change for later bodies, incorporating the plastic plug, and dispensing with the end flap on later light shields.
So, you can get this M2 specific light leak for two reasons. Either the early light shield is not totally effective at blocking stray light entering through the rf window on your specific camera, or someone did a CLA and replaced the early light shield with a later one, without adding the plastic end plug.
Best fix is to install the later plastic end plug, and just use the later light shield. Problem solved.
Credit to Don Goldberg for the photos and the explanation.