Googling about I found the drivers, probably like you did on the Vintage Apple Drivers website.

http://www.vintageapple.org/macdrivers/accel.shtml

But I also found a link to this forum of a Mac 512K using the same accelerator. The 50 pin head goes to a Kelly clip that goes to the 68K CPU on the 512K Board. Links to to pictures of the owner's board though flickr.com

https://68kmla.org/forums/index.php?/topic/503-gcc-hyperdrive-20-in-mac-512k-photos/

No luck on finding a manual - yet.