EDIT: I should add since you're using x-ray film, film speed and contrast could be all over the place under tungsten or daylight conditions etc. If you're doing your step wedge tests with tugsten light for example, I'd expect very poor speed.
The film is GREEN sensitive. Tungsten light is reddish which the film is not very sensitive to. I think the film speed test should be done in daylight with a neutral colored subject.
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