While I don’t really no, I, for one, could not dispute that the “M” stands for Ammunition, or perhaps for the Munitions branch of the company. Their name translates to English basically as “Czechoslovakian Munitions and Metal Working Factory,” so the “M” could delineate the name of the ammunition-producing section. I think it might be a very good call by Peelen.
I forgot, in mention their manufacture of charges, which may have carried the “M” marking simply because they were arms and ammunition-related products (it would be interesting to know, if they made any, how they marked metal products that has nothing to do with arms and ammunition?), that I have had the Czech-pattern 7.9 x 57 stripped clips intended for the Czech Versions, like v. 24, of the German Model 98 rifle and carbine, as well as those for the M88.