After doing much research, I think the Criterion may be the Perspecta mix and the MGM is the four track magnetic.
Perspecta was a mono system designed to be the poor mans magnetic and could derive two front channels out of the single optical track for a center focused LCR in the front of house. I saw a screening of Hidden Fortress this way recently and it seems rather like the FP mix. This film is reputed to have the best Perspecta mix of all.
In other words imagine matrix versus discrete audio and you start to get the idea.