This is the 3 coupler in my system counting away from the collector. It fits into a 4" metal pipe on the side facing the air flow and a 4" metal pipe fits into it on the other end. It has a branch coming out for a 2" PVC pipe. This will run down under a window and normally will be hosed to the Table saw although it gives me a convenient low Vacuum hose connection for small tools and odd clean-ups.
The author hasn't provided the model origin yet.