Here is an interesting monoshock swingarm setup found on Facebook Marketplace some time ago.
I've never seen this setup before, it seems to be a kit, possibly by DG or Profab in the 1970s. The swingarm looks to be professionally made, as does the spring system and the various mounting brackets. Damping is provided by separate shock absorbers. The system is shown on a Bultaco frame with the spring bracket added to the front downtube and laydown type brackets on the rear tubes for the shock absorbers.