[Leica] Strange filter...

[Simon Ogilvie <simon_ogilvie@ionica.co.uk>]

I was sorting through some old piles of junk yesterday
and came across an odd Leitz filter.  It's a yellow screw
in filter with a screw diameter of around 3/4 of an inch.
It's marked Ernst Leitz Wetzlar, Germany  2* on the
filter itself, and it comes in a tiny red box labelled
Gelbfilter 2, "Firmy" 3.5 - 5 cm.  I assume this means
it's for 35 and 50mm lenses but it's so small it won't 
fit any of the current range.  Is "Firmy" the code name
for it, and if so does anyone know what this was for?

Thanks,
Simon.