When in '91 was the engine gasket issue resolved?
 

Is their a month where all cars forward from that date were built with head and base gaskets? How do I check this by VIN number?

Head gaskets were installed after engine serial number 62M06836 (M64/01) or 62M52757 (M64/02 with Tiptronic transmission).

Do those numbers also match the VIN?

You have to rely on the engine number. There were some earlier cars converted to the later spec, either under Porsche warranty or by owners. We do a fair number of 964 of early and late spec, some leak, some don't I don't worry about it too much anymore.