Difference between the 3500 and 3700 is just the impeller. Infact both are labeled as 3500, think the new ones are no longer that way though. Neither has a venturi with the pump, the 3700 however has a needlewheel impeller.
http://www.aqua-medic.com should have the details.
Last I checked wai's didn't really stock parts - but if I can get by when he's open... (Just visited to see a closed sign) The part is basically a male - male adapter, male metric on one side, male mpt on the other, kind of looks like a top hat with a small o ring on the metric side. Screws into the outlet of the pump. Can't take a picture as I seem to have misplaced the part suddenly.
Aquamedic direct may not be much of an option - seem to recall their warehouse burned down just recently...