There is two types of air start systems on Ruston hornsby engines, but let me start from the beginning. Twin flywheel Rustons are considered more for agricultural/ farm use and single flywheel versions are industrial, the air systems are different on each. Look up Ruston grain elevator engine Mark CR/ 3XHR Mark CY/4YHR and you will see the air start valve on the head with a lever. That is the manually timed verision, also note on the side shaft side of those engine next to the governor the have a face machined into the casting but is not used on the twin flywheel model. On to the single flywheel industrial version ,one those there will be a cam bolted to the sideshaft that actuates an air valve mounted to the casting I refered to in the last sentence , from that valve there is a steel crossover line to the head and there is a check valve where it injects to the combustion chamber.
This a video of my single flywheel industrial engine with the cam timed air injection after I reset the timing. https://www.youtube.com/watch?v=VK8-9ofwM4M