The early years of anime doujinshis, published in very limited numbers by fan circles, still awaits scholarly review. But it is clear that Space Battleship Yamato was their top subject in the mid-to-late 70s. They served the dual purpose of giving fans a connection point and also sharing content that simply wasn’t available anywhere else (such as production materials rescued from Office Academy Studio).
Until we find evidence to the contrary, the very first Yamato doujinshi appears to be Negal, the journal of Space Battleship Yamato F.C. [Fan Club]. Note that this was NOT the official club that would stake out that name in 1978. It was a close race, since the work of “Cosmo Battleship Yamato Laboratory” followed just two months later. (Read all about the early fan clubs here)
To honor its position in history, the entire run of Negal is presented here in its own collection, separate from the (much larger) vintage doujinshi archive here. Some issues of the followup series, titled Neo Negal, can be found there.
See it here