As the name says a mask substitute filter has the same colour and density as a mean film mask, to be used on its own to compensate ideally completely for the missing mask. By this the values for the adjustable fliter will remain in the range one is used too.
Otherwise it could be intended as add-up orange colour on the adjustable yellow and magenta filter, but that makes not much sense to me.
Basically it can be of any construction:
-) minerally coloured glass
-) organically dyed gelatin or resin
-) dichroic coated glass (or resin)
But of course when usin dichroic filters for the rest of the filter assembly, it makes sense to use this type of filter here too. From the look of it, bi-coloured, orange and blue, it seems to be dichroic.
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