Anything of a technical nature.
-Product development (inventing)
-Physical Culture (1800s-1960s)
-Acoustics + Psychoacoustics
Uhh yeah, this could be a very long list...
Stanton ST-2, XXI, DP35/40/50 and 60
Pioneer SE-A1000 and SE-L20A
Sony DR-5A and 3A
Realistic Pro 20
Koss Pro 4 in green, Pro 4AA,'stereophones' in black, 1967+1972 Stereophones
Pickering PH4933 (green), 4955 (brown)
B&O Form 1
Takstar TS671 and HI2050
Philips SHP9500, X2, 5205bk, 5905bk and SHP2000
JVC SZ2000, S680 and HA-M55X
Sennheiser HD419 and HD439
Superlux HD681 and Evo
... various others, some 70 in total.