As far as I'm aware Kodak have discontinued their Masking film (along with a lot of other lab products). I've used TMax100 for contrast reduction masking of transparencies for Ilfochrome printing. You get a colour shift which changes your filter pack and with some developers this can be different between the highlights and shadow although it's not usually noticeable. For a starting point underexpose the film and develop in a dilute film developer (I used ID11 1+3) for 5 minutes. If you use a tungsten source to expose the mask and use daylight balanced film you may find you need to adjust the light colour to approximate daylight (or use a filter) although you can use this to your advantage to help minimise colour crossover. I find that for the majority of my transparencies which require masking, as mentioned in another thread, I can alter contrast during the Ilfochrome development stage.