The explanation for "Luftpresser" is quite simple. Starting (I believe) in the 1920s, there was a drive to replace "foreign" words in use by "proper German" words. This, of course, became an official policy in the Third Reich. So "Kompressor" (compressor) was replaced by "Presser" (translation unnecessary). So, "Elektro-Luftpresser" means simply "electrically-driven air compressor". You may well have seen other results of this policy in schematics or descriptions of German systems; for instance, in descriptions of electrical systems a frequently-used word is "Selbstschalter" which is a relay; the usual word "Relais" was too French to be used in official documents. In a similar vein, "Hydraulikflüssigkeit" (hydraulic fluid) was replaced by "Drucköl" (pressure oil). Such verbal shenanigans make it more difficult for non-German speakers to decipher official documents or, as in this case, identification plaques on parts.