correct

It retards from base timing about 8 degrees, I would guess by looking.

Advance portion works as normal.

Can be used as boost retard, but not really graphed for it, welding up the mechanical advance would help that too though

What happens when you put positive pressure in those ports? If it works as a boost retard that would be great

Yes, it came out of a Datsun B310 (USA model, certain ones only)

This type is more common in the mid-80s Nissans, like E15 engines.

Do you know what that distributor came out of DD?

A few of the US distributors have this. Vacuum advance unit as normal, but 2nd port retards the spark timing.

Do any of the Aussie nissan engines have this?

A little video, boring perhaps, but shows how it works.
video