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.
