Hi Marmite,sorry for the delay in replying it’s been a manic couple of weeks,I hope the picture helps though
The air pipe from the front carbs tees onto a straight pipe which connects to the rear carbs via the air pipe that has 4 connections,one for each rear carb,one for the front connect pipe and the fourth which vents to atmosphere as per the picture.
That is the vent pipe on the left as viewed from the rear of the carbs