I use a 4x5 Saunders LPL enlarger, mostly for medium format negatives. I've recently shot a bunch of 35mm and for the first time tried to use Saunder's 35mm diffusion module, which dramatically reduces printing time with this format. Unfortunately for some unknown reason the module creates a slight but significant vignetting on the left side of the image (using a 50mm lens). This suggests to me some misalignment of the module but after taking a close look I can't see a problem. Any suggestions?

Thanks, Arthur