Try this page:
http://www.tvhistory.tv/1950-59-HOFFMAN.htm
lots of Hoffmans, but I did not see the 21M721. If it has the yellow glass, it for sure is a B/W set. There is something odd here..."M" usually indicates mahogany... Hoffman used "P" for cherry ,"B" for blonde limed oak and "W" for walnut.
hope this helps,
jr