My 22KW Generac standby genset that powers the house, Tom and I completely disassembled it ( removed all the components from the original steel cabinet) and reinstalled them in a new from Generac aluminum enclosure I purchased from Generac. My original enclosure was steel and badly corroded, so we gutted the original one completely and moved everything into the new aluminum one. When pulling the components, generator head, engine and all the electrical and electronics out, all the electrical and electronic components were marked 'made in China or Taiwan' and I also adjusted the valves while we had the generator-engine out of the cabinet and when I removed the valve covers, inside the die cast valve cover's casted into each one was 'China' on the inside and interestingly, all the socket head fasteners on the engine as well as all the bolts securing the engine-generator head to the new chassis were metric, not SAE. Keep in mind that my 22KW as well as the John Deere 32 KW standby that powers the shop, both are over 20 years old.
I go with the 'assembled in USA from parts sourced worldwide'...
Never had the diesel powered one out of the cabinet, nor have I ever adjusted the valves, but I did on the 22KW air cooled unit for the house, so we had the opportunity to 'examine' that one close up.
Each unit has it's own vacuum actuated transfer switch but I've never been inside either of them, no need to as both generators and transfer switched operate flawlessly and have for well over 20 years now. Basically, all I've done is routine maintenance and replaced the starting batteries.
I'd say both mine were assembled in the USA with parts sourced worldwide, mainly China and Taiwan. Neither of my units state anywhere, 'Made in USA'.