Thanks Hugh. I'm dealing with 'high-pressure (HP) sodium' lighting in the streets of Canberra.
There is significant commentary on US forums regarding this filter against HP Na lighting. Here's a useful link that I came across:
http://www.northsouthsky.com/astronomik-cls.php
The 2" option is ideal for my Tak extender TOA 1.6x setup, as it will not change the metal back distance of extender to the CCD chip, and they don't make a 'clip in' for the full frame chips at this stage (to my knowledge).