His golden era with Packard was certainly the 20ies (not that he was ever a Packard employee, he only had employee status with Chrysler - where there was a lot of infighting with engineers and he was eventually sidelined - and Checker as automakers in his career I think). But through his various design companies he also provided individual designs on a project basis to Lincoln, Ford, Checker and Packard, even as late as the 40ies.
I never knew he even did designs for Mercedes-Benz!
By the time he worked for Checker in the late 30ies, he had left behind that lavish 20ies design language he was initially known for.