Hi Jim -
Never heard of St. Macaire, either. There is (was?) a Cotes de Bordeaux-St Macaire appellation for sweet white wines.
Anyway, I looked it up under the Vitis International Variety Catalogue (
http://www.vivc.de/), which lists it under "Saint Macaire" as a black grape with the synonyms Bouton Blanc (even though it is not a white grape); Macau, Moustere, and Moustouzere. The database doesn't give any other info.