I think that valve mechanism is designed to control fuel flow while maintaining a relatively constant jet velocity, and providing a needle type orifice that can be cleared by cycling the valve. The construction isn't clear, but I'd guess the vertical needle has a slot that is engaged by a fork threaded onto the end of the reach rod connected to the valve handle. The advantage of this construction is that the valve stem seal is outside the boiler next to the valve handle and is thus kept relatively cool.
here are my thoughts on the design of the valve adjustment mechanism. The adjustment slide features an inclined square section that slides within a corresponding cutout in the valve, thereby opening and closing it to varying degrees.
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