Guy is selling these as "M961"s but the head is obviously larger and to me the body looks longer, maybe 12 volt/4 batteries?

Looks like an M962 variant, but about a battery's length longer.

I think it is a M982XM00. "M4 dominator" "turbo head".
Second question is: what bulb does it take?

With four CR123 batteries:
MN60 - 225 lumens for 1 hour
MN61 - 350 lumens for 20 minutes

Or, if you run two 17670 lithium ion rechargeable batteries instead, you can run this:
MN15 - 125 lumens for 1 hour

It looks like Malkoff is still making the MD60 replacement drop-in that should kick it up to 650 lumens out the front for an hour and a half. There is also a cheaper, less bright and non Malkoff LED drop-in available from Lumens Factory, the M3T-LED. Or if you want to stick with an incan bulb, Lumens Factory also offers the HO-M4A which is claiming 550 lumens for about half an hour - although personally I wouldn't trust it on a weapon light.