Moates.net seems to call the mask "the operating system" -- from my understanding this is not the best term but it did help me understand what "mask_id" meant just a little bit.

I would say two things (based on my novice understanding)…
1) The EPROM contains a bunch of tables, the mask_id is a way of keeping track of which tables it contains and WHERE they are located in the EPROM
2) The EPROM ALSO contains actual microprocessor code (a program that uses the data in the tables). The code in different "mask_id's" is different.

So the mask_id is like a "model number" for the data in the EPROM… which is both code and tables.