As far as I know, there have never been plans for 6-car Mk IIs.
Maximum length for Mk Is is 6 cars (which we see in the system during peak hours).
Maximum length for Mk IIs and the new cars for Evergreen Line is 5 cars (which don't exist anywhere in our system yet).
The difference between the Mk IIs and the Evergreen Line cars is that the Evergreen Line cars aren't two married pairs, but rather a single continuous link of 4 cars.
Code:
Mk IIs
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Future Evergreen Line
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Mk IIs (with future 5th middle car)
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