im pretty sure thats a standard old Macintosh monitor, which means its pretty much VGA but with a different plug.
i could be wrong about this but i think the only 15KHz rgb monitor Apple made was a 12 inch intended for use with the Apple IIGS computer that isnt too good anyway (the tube dotpitch is a little too coarse for my liking) ; I think all the old monitors are composite only because thats all the Apple II's give except for the IIGS's and later models are all VGA-like Mac monitors?