I have a few Multitank 6 drums (marked as Multitank 6 and 2830), they all have the ridges. My Multitank 5 does not. I process all my B+W and C-41 8x10 in the Multitank 6 with excellent results. I did extensive (and expensive) testing to see if there were any ill effect from the small ridges on the back side of the film and found none: neither visually on the negative, in the resulting print, nor densitometrically.