The third picture is a high flow valve. Most industrial regs come with them, and they're useless to us. You can junk it.
The outlet is 1/4" NPT.
The inlet is either 1/4" NPT or 1/2"-27, but you won't know until you remove the one that's on there, which may or may not be necessary. My guess is that it's not possible to slide the nut down the nipple - I have one or two like that - so you'll have to use a pipe wrench or locking pliers on the nipple shaft if you want to remove it. As far as the plastic gunk in the orifice, you'll need to remove it. It's probably the remnants of the last nylon washer it was used with, that somehow got down there. If you can't remove it, you'll need to replace the nipple. If you can remove the gunk, and you don't find a leak along the nipple, then you're fine and don't have to worry about removing the nipple.
By the way, the "weirdly threaded monstrosity" that the inlet threads probably are is a simple fix, if you even need to go there. Western Enterprises part CO-7, I think, is the 1/2-27 threaded nipple, and costs all of a dollar or two more than a standard 1/4 nipple. They both use the same nut.