Simon.
I guess we have to reconsider the whole thing. We have located the HP supply regulatingvalve, HP filter and the 205>30 kg/cm2 reductionvalve all placed on starboard side. Between these items are 20mm pipes. Out from the reductionvalve I have not been able to ascertain the pipedimension, but looking at the valvehousing I doubt if it is a 40 mm pipe out from this valve so I assume 20mm. We know that to and from the main startingvalve at the maneuvering stand the pipes are 40mm, thus the pipe from the airvessel should be 40mm.
The pipe at the forward end of the airvessel shown on your photo is definitely a full 20mm airpipe going upwards, hardly a drainpipe, the smaller pipe which can be seen could be a drain. I have not been able to locate any crossover. Unless you have any photos showing otherwise my proposal would be: The supply to the starting airvessels from the reduction valve on starboard side is 20mm, entering both vessels at lowered forward end, which means that the crossover is a 20 mm pipe. The separate airsupply from the starting air vessel to each engine is from the aft end of the vessel (as you was asking) to the main startingvalve on the engine. It is no 40mm crossconnection. Each engine takes its startingair from its "own" vessel by the 40mm pipe from the airvessel aft side, the 20mm pipe is only used for topping up (filling) the airvessels. The lay out is based on a direct reversible engine which has a large airconsumption (you have to restart every time you change direction of revolutions) thus the capacity of the airvessel should be for at least 10 starts before topping up.
Tore