[Leica] 90mm lens question

[Simon Stevens <simon@camera-craftsman.com>]

I have a couple of questions for the more historically-knowledgeable
members of the list (which I certainly am not.) I have a 90mm f2
Summicron SSN 2203xxx. It's one of the larger black Canadian models with
2 stage collapsable hood, tripod mount and detachable head. The optical
quality is very nice indeed with a (to me) lovely bokeh. I have seen
several similar lenses from roughly the same time period (I belive it's
from the late sixties) but I have never come accross an identical one.
Mine has a preset ring in addition to the regular aperture ring. None of
the others I have ever come across, or seen pictures of, has this
additional control which obviously makes it ideal for viso use.

Stephen Gandy's site mentions that all of the 90's were made in
relatively low numbers, but how common or unusual is this version with
the pre-set?

A second question is more about use. The filter diameter seems to be
48mm. Is this standard pitch? B+W seem to make filters in this size but
I'd like to know before ordering any whether they would fit.

Thanks!

Simon Stevens