Photographed with a Canon EOS 20D and an Optomax 35mm f/2.8 lens from the screw-thread M42 era.

I don't think Optomax made lenses; like Vivitar, they specified what they wanted and got manufacturers (usually, if not exclusively, Japanese) to bid for the business.
There's not much on the web about the Optomax 35mm f/2.8 lens. A person on a Pentax users' forum reported in October, 2010, that he had bought one for £4.99 on eBay. He said, 'Like all lenses of this period, it is all metal and very well screwed together. Image quality is quite good - the best so far, in my rapidly growing sub ten quid lens collection.'