Anyone know what this filter is for?

Presumably the same benefits a color compensating filter, like CC40, would yield. This the link to what I was referencing: https://micro.magnet.fsu.edu/primer/photomicrography/ccfilters/ccmagenta.html
Granted this isn't for certain, but its possibility.

AgX said:

What benefit should a magenta-filter yield at "intense tungsten halogen lightbulbs" ?

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Yeah, I looked in my 1971 Kodak Filters book. No mention. Makes me wonder if this was used in a scientific application?? Maybe to balance a odd light source or maybe aerial work ??

It looks like a Wratten #32 filter.
My 1940 Kodak Reference Handbook has an extensive section on filters, but there is no "MC 1" filter listed therein.

Howl23 said:

Presumably the same benefits a color compensating filter, like CC40, would yield. This the link to what I was referencing: https://micro.magnet.fsu.edu/primer/photomicrography/ccfilters/ccmagenta.html
Granted this isn't for certain, but its possibility.

Click to expand...

But for halogen-lamp lighting and daylight-film basically the blue filters 80A or 80B are used...
However, in microscopy one already starts with a heat-protection filter making the light more bluish. Then the microscope lenses again absorb more light in the blue end than normal lenses, which may partially counter the former effect. And then there is the binder a specimen sits in etc. Somewhere in this that magenta filter might come in... though not very convincing to me.

However it may be directed at the staining of the specimen. In this context today Didymium filters are used. That filter may have been intended in this sense.

And in this context "M" might refer to "microscopy.