Inching backwards in time, here's a nice color doujin cover from 1992. This doujin was just in time for Comiket 43. Comiket goes back to 1975, so apparently doujins were around in meaningful quantity before 1975. But looking at the oldest anime videos that
bakabt.com has (circa 1969), I cannot find any corresponding doujins from that time period. It's the mysterious impenetrable past with dinosaurs still wandering the earth and doujins becoming extinct sinking into the mud never to be found again. My very oldest "doujins" if you could call them that were terrible photocopies purchased from enterprising people in Hong Kong photocopying some japanese doujins they got a hold of. But I was desperate back then due to the difficulty of finding ANYONE in Japan that would send stuff stateside. I guess attitude went hand in hand with the open mallshops selling copies of western software products and appliances.
