Promaster still exists:
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They have sold many different OEM lenses in the past, primarily rebadged Tamron. However, the lens shown in the OP apparently is an Osawa MC 1:3.5-4.5 28-80mm Macro, which was also sold as Bell & Howell 1:3.5-4.5 28-80mm Macro. Osawa was a Japanese trading company that went bankrupt in the early 80ies, one of the very few examples of a large Japanese company going bankrupt. Osawa was somehow affiliated with Mamiya - most of the Mamiya 35mm lenses were available with other mounts under the Osawa and/or Bell&Howell brand. The end of Osawa also meant the end of all 35mm activities of Mamiya, some time around 1983.