Why is it all secretive? Paul from the PowerWashStore recommends rebuilding or replaced the check valves for the bleach annually. The back panel in the Proportioner has bolts on it. Surely people who have purchased it have taken it apart and seen the internal components, right? But even if it was somehow kept secret, there is really only so many ways to accomplish the mixing. And when you have 3 lines, 1/2" inlets, 1/2" outlets, 3 GF 523 metering valves, a compact footprint, and hastelloy check valves, there is really only one or two options to build it.
Do you think the stainless steel enclosure is solely for “keeping the plumbing a secret”?