The stein may have been made by a company *in* Mettlach, but it was most definietly not made by the company known as Mettlach (a term which actually refers to the factory of Villeroy & Boch in Mettlach). Or, the stamped mark may be completely fraudulent.
I have seen this style of porcelain stein on ebay several times. V&B Mettlach made stoneware steins. Gerold Porcelain probably made this stein. They sold blanks to decorators according to the firm that bought them out. This is the style of stein favored for the reproduction reservist steins sold in the 1960s.