Sorry for the late reply - been away for a couple of days.
That's the one - a restricted orifice less than 1/8 in diameter in what looks like a standard fitting.
Although the breather connections are different between early and late B18, and B20, there is absolutely no problem mixing and matching.
I'd also advise not using the thin round air filters, but using the late three bolt style and connecting to the hose fitting on the front filter. The "plate" used between the filter and the front carb is a waste of time - they are invariably plugged, and don't pass enough air anyways.