The Xenon on the Retina IIa frequently shows haze. I used to think this was due to a deterioration of the cement layers in the front and rear groups, but I eventually took an affected lens, separated the cemented elements, cleaned away the remaining cement, and found that the haze was actually on the the surface of the lens that faces the shutter.
The haze does not clean off with any solvent and is most probably the result of a reaction of that particular type of optical glass with the atmosphere, and/or with the out-gassing solvents from lubricants used in the shutter.
So... for the Retina IIa at least, there is a strong case for hunting out a Rodenstock-lenses version to use instead.
Unlike with the Retina IIIc types, where Rodenstock lenses sometimes show separation in the cemented front group.
Regards, Chris