I do not recall ever using a diffusion screen/glass in my M600.
But I did use a PH211 photo bulb, which should be evening out the light itself.
I concur about the red filter. It swings all too easily and it does come in from the side, so could be causing your problem. If you do not use it, I suggest taping the knob, so the red filter is not accidentally move into the light path.
Ahhhhh (light bulb goes on)
The negative carrier of the M600 has movable masking blades. I think the one on the left side might be mispositioned, and is slightly in the light path. Move ALL the masking blades OUT, and I think your problem will be solved. I never used the masking blades myself, as they were too far from the film plane to create a sharp cutoff.