More info on designations can be found here:

http://flashoffroad.com/features/Trivia/trivia.htm

(Look under "Hummer History - UPC/ Model codes i.e. HMCO")

HMCS = HuMmer Commercial (or Civilian) Station Wagon

The H1 Alpha models (not listed in the link above) are as follows:

HMDO - for HuMmer Duramax Open Top
HMDS - for HuMmer Duramax Station Wagon