Need help with a fitting on a Holley 2 Bbl carb, please.

Working on my good friend's Australian '71 Valiant VG (think '69 Dart for the body), with a 318, 904, and a Holley 2 Bbl carb, and I'm down to the final punch list. I think....

Bazza (the owner) would like to keep the car as OEM as possible, so with respect to that, what is supposed to connect to the nipple fitting coming off the top of this carb? Looks like a bowl vent to me, but still- where does it go?
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I've looked through the Aussie Shop manual (nada), the carb kit (ditto), and all of the pictures Bazza has from Day One on this car, and I still can't find what this is supposed to connect up to.

And I'd rather not leave it open and just slap the air filter on top of this and call 'er good.

Is anyone able to shed some light on this?

Thanks.

Jim