Senator is probably the largest pen manufacturer that we know next to nothing about. Today, Senator, properly Senator GmbH & Co., is a major German producer of office items and ball point pens, particularly promotional pens in a wide range of quality. However, its predecessor firm, Merz & Krell, was very well known.
Merz & Krell was a producer of fountain pens in Germany since 1920, particularly the Melbi brand from the 1920s-50s, but in addition to their own brand, Merz & Krell was a major producer of parts for Montblanc, Pelikan, and other brands from the 1930s forward. Indeed, Merz & Krell are probably best known for their Pelikan 120, produced under license for Pelikan from 1973-77; they look like true 120s but are a touch longer. In later years, their Diplomat and Senator brands, each with a range of piston-filled models from student to full size, were merged into the Senator brand, and in 2006 Merz & Krell changed their corporate identity to Senator.