Maybe so... Rössler says around 1955, which is way off. I have argued elsewhere for a pre-Respirator date of 1967, based on a number (67017) on the case back (below) and my own notion that Zenith shouldn't have been making "Compressors" (though they seem to have made only a handful) and "Respirators" with EPSA cases at the same time, but who knows. 😀