Murray - I have 2 Mockba-5's, one came set up properly (except for a light leak around the catch on the back which was easily fixed) and the red window doesn't give any trouble with color film. 6x9 chromes rule, I don't miss not having a 6x6 mask.
The 2nd came with lots of problems, all of which were relatively easy fixes, but the rangefinder is out of whack, and THAT my friend is a bear of problem to fix. Do NOT take the front rangefinder apart or mess with it unless it's absolutely necessary. I still haven't gotten mine figured out. A slight adjustment isn't too bad, but once it's taken apart you're in deep, uh, fertilizer.
Matt - it looks to me like that's your problem, not the lens. Make absolutely sure the focus is properly set for infinity. Just my 2 cents.