Acoustic valve.
Mines a Y reg aswell.
Pretty sure its just a breather hence its little importance when they decided to leave it off the later ones of the same model
It connects to the fast idle valve responsible for your automatic 1500rpm at cold to warm the engine up. At least on the ph2 i found it connected to a fast idle valve, its bolted on the side of the acoustic valve and has a 2 pin connector going to it.
Samuel