Just received a press on lens hood for my 135 Optar in the mail. It came with a filter that I was not expecting. I guess the selling just threw it in with the sale.

The filter is a drop in Wratten series 6. Looks light ornge or yellow. Could have faded over the years.

Anyone know the filter factor? Is it a good filter to leave on the lens for black and white flash shots? I guess it will lighten skin tone. I will just have to experiment with it. Anyone use it for people shots?

Well the series number refers to the diameter. Originally it went from series IV (4) (for cine cameras) to VIII for large lenses typically for an 8x10 camera. After some time, Kodak came out with series IX for their Wide field Ektar and other lenses. Still photography abandoned the series system when Kodak did, but apparently it's alive and well in the cine filed with a Series X and several half series now (found this out while Googling for another post, damn if I can find the site now)

The most typical yellow filter will be K2, which is now called a #8 and is supposed to bring Panchromatic film (any B&W today is Panchromatic) in line with what the human eye sees. It's also great on overcast "steel grey sky" days as it cuts a lot of excess blue.

Filter factory 1.5 to 2 stops.

Light orange or dark yellow smells "G" or #15.
Darkens blue skies a very big lot, removes haze from far away scenes.
Filter factor 2.5 with Tri-X sheets in © 1976 Kodak Master Guide.
BTW Wratten filters have markings stamped on the edge of the metal ring.

General rule of thumb for filters and factor:
Set your meter to the film speed or EI you plan to use, point the meter to an evenly lit monotone such as a cloudless section of sky, take and record a reading. Place the filter over the light meter's sensor, take and record another reading with the meter pointed to the same place as the first reading. The difference in stops is your filter factor. This assumes you are using a reflected light mode/meter.

I took a closer look with a lupe and saw the letter G on the filter rim. Looks like it faded abit since it is light orange, not dark. How would this filter effect black and white shots on 4x5 sheet film with Plus X film using my pet Press 25 bulbs?

It helps to know a little bit about color theory. The primary colors in light are red, blue, and green. The secondary colors are magenta (red + blue), cyan (blue + green), and yellow (red + green). A red filter is red because it passes red light (the red part of the spectrum, which contains all the colors) and absorbs blue and green. A green filter absorbs red and blue, and a blue filter absorbs red and green. A yellow filter (yellow = red + green) passes red and green and absorbs blue. Whatever the filter absorbs looks lighter in the negative, and darker in the positive (print). Therefore the effect of a yellow filter in the print (positive) is to darken blue and lighten red and green. A red filter renders blue and green much darker in the print, and yields dramatic contrast between blue sky and clouds. The blue-sky darkening effect of the yellow filter family is less pronounced than the red. A yellow K2 (no. 8 ) filter yields correct rendering of sky brightness on panchromatic films; a yellow G (no. 15), darker than correct; a red A (no. 25), very dark; and a red F (no. 29), almost black. (Data from Kodak Data Book AB-1, "Filters for Black and White and Color Pictures" [1969], p. 18.)

In the bc 60s (before color), because of budget and technology constraints, TV shows routinely shot 'simulated night shots' by shooting during the day with a No 25 red filter and under exposing a little, sometimes they added a polarizer

Of course, a polarizer is no good for panned shots, as the effectiveness of the polarization changes relative to the filter's angle with the light source. But another commonly used way to simulate darkness in a day-lit scene is to use a neutral density (ND) filter; these come in various strengths, and are equally effective no matter the colors in the scene.

For the Wratten G (#15) filter, 1 1/2 stops more expsure will get you in the ballpark. When these filters were new, they had a slightly different factor for each and every film. 1.5 stops is what I use.