The HyperDrive was an internal hard drive sold by General Computer that was added to many early systems. Apple didn't include any hard drives internally until 1987 in the Mac SE model.

Check the thread called "512Ke Rare?" for a nice list on how to tell if your 512K was ever upgraded to a 512Ke. It sounds like your Mac is a 512K with a few upgrades/mods.

Do you have any pictures of this machine?

PS: Welcome aboard!