Thanks for posting the question; I'd never heard of Simmonds brand injection before. Sounds like it works like the 'Continental' system; is that the actual brand used on the medium/large Continentals?
Simmonds Injection for light aircraft was mostly used on the Beech Twin Bonanza and Queen Air aircraft. GO480 and GO540 engines.
I only found one company that overhauls these units.
Simmonds is still listed in Norwich NY. They were originally a British Company.