Scanned Document Links:

Aiwa “Sound Selection” (1988/10)

Technics / National “Comprehensive Headphone Catalog” (1984/2)

Sony “Headphone Comprehensive Catalog” (1984/10)

Sony “DIGITABLE 5 — FM/AM Stereo Radio Cassette ZX-5” (1983/8)

Other Links:

Head-fi.org (forum platform discussing earphones / headphones)

Archive.org (hosts an abundance of archived vintage catalogs)