That is one really good question magnum05!
As best I could determine from spending my Thanksgiving evening doing internet research, the best I could come up with was that "swivel" and "hinged" are two different terms for the same Harris bipod function?
A partial description as an example from Midway:
These are the models with S in the designation
Hinged base has tension adjustment and buffer springs to eliminate tremor or looseness
Available for all models except H models
Someone may have the correct answer if that isn't right and I'd like to know as well.