Both the Schneider Series 9 81-One and the 81-Two Warming Color Conversion Water White Glass filters are designed to add warmth to skin tones and reduce excessive blue in outdoor open shade.
It's Autumn. The clouds are low hanging and the sky is "patchy blue". You've gone for a ride upstate to the mountains and the light in open shade is bluish. A photo taken of your girlfriend with daylight balanced film leaning against the side of a barn will yield bluish tones that she might not find particularly flattering.
An 81 filter will replace some of the yellow missing from the image and bring her skin tones back to a more appealing level. The 81 series allows you to choose how much warmth you wish to replace in the image.
Note! The 81-One is more appropriate for a subject with darker skin tones
Series 9 81-2 Warming Color Conversion Water White Glass Filter