You are correct, the reflections of the bellows are showing up when the negative holder is not in place. I have tried masking the stray lights out, but I have not been happy with the not-so-good Dmax I am getting with this enlarger because of the stray lights bouncing on to the paper. My prints from a Leica Focomat V35 are much contrastier in comparison. This is what led me to do the investigation.
How are you determining one enlarger prints with more contrast than the other? Same lens on both enlargers and graded paper, right?