The only set of clubs that i know of that has an E club, are clubs manufactured by Ben Hogan. The E club is the same or another name for a pitching wedge. According to the Ben Hogan website, it is called the 'equalizer' it is called so because Mr Hogan believed it completed or equalized a set of clubs.
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I've seen it on TaylorMade irons for "Attack Wedge", which I think is the same thing as a "Gap Wedge".
While TaylorMade may dub their A wedges "Attack Wedges," an A wedge is commonly referred to as an Approach Wedge.
Are you sure it's an "M"? It's possible it's actually a "W", some clubs have a "W", which stands for "wedge" which is usually a wedge that has different "lofts" within the same model.