Nice mark photos. Sorry to say, the holder of this mark is presently unknown. There are thousands of Mexican smiths who's work is known but who's names remain unknown. Up until the 1970's, record keeping, in Mexico's assay offices, was a pretty spotty affair. Most of the names we know today are because the makers became well known during their productive years, due to the high quality of their designs and better craftsmanship in their workshops. Of course, there are plenty of great pieces by unknown Mexican smiths and you may well have one here.
I've handled a fair amount of their pieces and know that "AE in M" has produced some pretty good work. The mark is sometimes referred to as AE in a heart, but after you've seen various stampings of it, it becomes clearer as an M.
Here's what I can tell you about them. The mark is usually seen in conjunction with Eagle #23, so they were producing between 1948 and the mid to late 1960's. Apparently they were fairly successful, having purchased and registered there own eagle number rather than using the generic #3 of the Taxco assay office.
Some of the work is made from heavy sterling, but they also made a lot of light gauge stuff. I'm guessing that the heavy gauge pieces are earlier work and, as time went on, they began to cut corners in production. The technique of your piece is called silver overlay; sheets of sterling soldered atop an oxidized sterling background. If you polish the high parts and leave the background dark, it will show a strong graphic contrast, as it was meant to.