The movable part of the head, that you lift with the red bar, does not sit perfectly on the negative carrier. So I think it is like 1/16 or 1/32 of an inch high. It did not seem to make a difference, so I never bothered to look into why.
Well it "might" be because I replaced the glass on the negative carrier with the aluminum glassless plate, and I put an anti-newton mask over the condenser on the top.