Ultrasonic is as good as the media penetration properties are. Heat expands the material different for aluminium and brass. I would try boil, or at least heat up the carb submerged in, maybe water is enough and maybe cykled with cold water. But I would try repeatedly 5 minutes in hot ATF oil maybe spiked with a little acetone or perhaps white spirit, something that lowers the surface tension even more. Then chill in fridge cold mineral spirit.

Ah, forgot the body is brass, but the idea is to have the material expanding and contracting and let the oil creep in. So heat it up and plunk it in the cold spirit and heat up again and repeat. Oxide has a different expanding rate and hardness and hopefully crumble to dust so it may work. 

Actually threading a -very short- bit into the bottom of the jet would not harm anything since the bottom hole has nothing to do with jet size or flow of gas like modern jets. The flow of gas entering the nozzle "jet size" is determined by the adjustable high speed needle and if you open that up fully the engine is flooded so the hole is far bigger in the beginning than needed for the consumption. Other things in the nozzle is important for the froth of air-petrol mix entering the venturi. The mixing starts at and with the small air-mix holes at the side of the nozzle. Every position and size has impact in the motoring, the size of the hole at the upperpart putroding into the venturi is important, the height of the nozzle is important and should be just below the center of the venturi. The rim should be horisontal and sharp. That's important. Even the outer shape, body volume taken up in the carburator stem has effect in the function. But not the bottom hole size (to a limit and depth) threaded or not! 

Good tip.  The point here being really gentle, have seen  corroded nozzles bend using this method. Then it gets really messy. 

I notice a problem now.  Carl states that the top of the nozzle should remain beneath the venturi and not potrude above the venturi surface.  Mine juts well above the venturi nozzle opening, by atleast a quarter of an inch.  So therefore, I assume it has been fitted with the wrong nozzle, or perhaps, they just needed a holder to store the nozzle and used this body, to store it, years ago.