Chris,

That's some kind of regulating valve assembly. The top on will be a gate type valve, only a quarter turn from off to full on for coarse adjustments. The bottom will be a needle valve and have several turns. You set the top one about where it needs to be then throttle with the bottom wheel.

What it does is hard to say out of context. You need to control three things to run a steam engine, the water going in to the boiler, the heat in the boiler, and the rate the steam is applied to the pistons. If you are smart you also have over-pressure relief on the boiler and mechanical brakes. My first guess would be it controls the rate of water entering the boiler. The firebox is coal and you regulate that with a shovel. Your pistons and a waste gate with gages need to be right in front of you so you can fiddle with them all the time. To the left of the boiler is a good out of the way place for the water inlet.