I was thinking the bellows kind of sealed within the housing, exposing and closing off the inlet and outlet ports, similar to a 2 stroke piston and cyl.
Nope...the bellows is entirely contained within the upper piece of the pump body. The inlet and outlet ports
are in the bottom part. So your second thought that it expands and contracts like a concertina (mini accordion) to generate the air need to push the flush to the tank or thru-hull is correct.
However, it's possible that diameter of the newer pump housing is also slightly smaller than yours, so I suggest you have a chat with Dometic/SeaLand tech support about it: 800-321-9886
Fwiw, I've written a piece I call "VacuFlush 101" that explains how it works (an amazing number of owners think they know but don't) that I'll be glad to send you if you'd like to send me a PM that includes your email
address (no way to attach anything to a PM).
"If you can't explain it to a six year old, you don't completely understand it yourself."