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Just found an old glass Wratten filter which is just marked A and "A In-cell" on the box. It is red but, but does it have any special use above the obvious use of a red filter? Wrattens I have seen typically have a number association. Exposure compensation seems to be about 2 stops.
The Wratten 25A is a deep red filter with a factor of close to 8. Perhaps that is what you have.
The 'letter' descriptions for filters are a method from long ago, pre-WW2 for sure. Kodak shifted over to the number system when the letter code became inadequate. From the book "Kodak Filters" I find that an 'A' filter does correspond to a Wratten #25, so you have the standard red filter for b/w photography. "In-cell" is a mystery though.
Thanks Mark, that pretty much confirms my assumption but I don't have a 25 to compare. I guess in-cell is another phrase for in-glass. I'll have to go out and try it.
Wratten "A" or "No. 25", as mentioned in "Kodak Filters and pola-screens" from 1946, is one of the three filters for color-separation. The others are "B" or "No. 58" green and "C4" or "No. 49" blue. "In-cell" filters can be used inside process-lenses, often together with a waterhouse-stop.
Just to add to this, here are the other old designations and their current Wratten numbers:
K2: 8 (yellow)
K3: 9 (deep yellow)
X1: 11 (yellow-green)
G: 15 (deep yellow-orange)
F: 29 (deep red tricolour)
C5: 47 (deep blue tricolour)
N: 61 (deep green tricolour)
There are other letter designations for filters.
Best,
Helen