Martin -
Of course, all self-clean machines were fitted with a two-port pump except for the Kenmores with the detergent dispenser. There is no denying that.
The WP you have with capped-off third port has a replacement pump if it has an under-the-tub disk filter. The original pump was the same, but without the third port. That pump went NLA shortly after BDs were discontinued, in favor of the 3-port with the cap and clamp. Yes indeed, if you un-cap that port, you can do any number of things with a hose added to it, including adding another filter, or making a fountain.
That said, if you put a 3-port pump into an older machine like your other WP with the extra hoses and the flapper valve, none of that will be useable as the 3-port pump is mono-directional, it DOES NOT reverse flow.
If you take that same older WP of yours, and add a 4-port to it, you have to supply the third and fourth (lower) ports somehow. The original filtration system would work fine using the top half of the four port pump, but without a water supply, the extra two ports would be dry. You would have to tap into the water from the flapper which is already going to cabinet mounted filter.
This is what I was saying about this being possible, but not immediately easy.
G[this post was last edited: 2/13/2012-18:27]