This article was published on how to determine the MOM version you are running http://support.microsoft.com/?id=923105

In brief, the build numbers for MOM releases are:

MOM 2005 Service Pack 1: 5.0.2911.0
MOM 2005: 5.0.2749.0
MOM 2000 Service Pack 1: 4.00.1300
MOM 2000: 4.00.0.066