Degree abbreviations vary from college to college, but many are the same. For example, in the United States "B" always stands for bachelor and "M" always stands for master. Some universities use the Latin name for the degree, which may cause the abbreviation to be reversed.
One possible interpretation, although it stretches the bounds of complexity, is that 'master' can mean an unmarried young man i.e. bachelor, and that this word master has then been clued as the academic MAster