Is this an early M2, (or perhaps a later one which has been serrviced at one point)? If the leak you are getting consistently looks like the one in the attached photo (elongated triangle which always crosses the boundary between two frames) I actually might know exactly what it is and where it is coming from, but might want some additional information. The color vs. black and white is probably irrelevant, but is it happening on higher speed film, and is it only showing up between frames which were shot with extended time periods between the frames?
Have you had the M2 a long time with no problems before now?
Let me know. I will check back tomorrow, and see if it sounds like what I think it is.