That said, there are some very high end manual toilets--W-C Skipper, Groco
EB and Model K--that don't have the short piece of hose connecting the
pump to the bowl...the only place TO put a vented loop in the intake is in
the line between the thru-hull and the pump. However, the design of these
toilets also makes a vented loop in the intake unncessary.

Thanks Peggy, that answers the question I was going to ask. I have one of
those manual types, I can't remember which right now and I'm not at the boat
to look, but it has a spring loaded pedal that you step on to flush. When I
purchased the boat, the surveyor wrote it up as needing a vented loop and
the insurance company dutifully required it, I could see no way of
installing one so I replied back that it was not possible or needed with my
head. Also the PO had conveniently installed second shutoff right alongside
of the head that can easily be reached even when sitting on it. I use this
religiously. The insurance company (BoatUS) never replied back so I suppose
they are good with it.

John