Finally found it !!!
After turning my loft upside down, twice - I stumbled across it in my shed today looking for something else entirely.
NOS its accumulated a little surface rust but should still be good for years yet, being such a state of the art design
I might not be sticking with complete originality, the bulbous ends were always a complete pain to access and squeeze
I'm unlikely to find another decent air box as they all rot out due to a lack of squeezing, so needing to come up with an alternative that baffles the upwards air flow but still drains the water away.