Today I had a look at my lenses -I mean photo lenses- discovered that my
Pentacon 2.8/29 was 55mm in diameter when I had read somewhere that it was
49mm and have written so in my database (maybe 51 nor more).
The Pentacon 3.5/30 is indeed 49mm and I thought they were very similar.
I had nobody to tell and thought about telling to my partner, but then I
said to myself, who in the world is going to care if the bloody lens is 49
or 55? and keep it to myself :-)

Sometimes things which interest or surprises us doesn't mean a thing to the
world around us. By the way the 29mm seems to be a 7 elements in 7 groups
lens while the 30 is 5 elements in 5 groups. I have read that the latter
design presents more problems than the former, more modern lens. I don't
know, the first images from the 30 look better than the ones from the 29.
Just in cases there's an optics freak reading me.