The .MAS lump is the old Master Palette lump. It stores a single master palette in 6 bits per colour component. It has been replaced with PALETTES.BIN, which can store a variable number of master palettes, at the more normal 8 bits per colour component. New RPG files still contain the .MAS lump, as it might be required for old versions to even show an "Unsupported version" warning.
It begins with a BSAVE Header. Then, there are 256 records of the type described below, totalling 1536 bytes. There may be extra bytes at the end of the file, so that the total file size is
7(bload) + 1536(records) + 7 (empty) == 1550 bytes.
|INT||Red (in range 0-63)|
|INT||Green (in range 0-63)|
|INT||Blue (in range 0-63)|