Unusual. I am not a pro, and I have never seen something configured like that. The spout opening and the handle are pointing the same direction. What is its mounting position (just curious). The only way that makes sense to me is if the long thread descends from above.
I think that does not use a cartridge, but instead would be a more classic design with a washer and seat.
You probably want to wait until somebody who may recognize that valve, but I think that you would want to fully open the valve, as if you are trying to pass water out of the spout, before trying to unscrew the brass. You can blow air to make sure you turned the right way. Then you will chuck that brass part into a vice, and turn the spout. You might stick a screwdriver into the spout for leverage. One hand on the screwdriver, and one on the spout.
You might start counting the flutes on the brass, because you are probably going to need that info to find a replacement stem.