During my browsing, I've run across mention of the Emerson "Tape Pac" 2000 series, a half-inch cartridge tape system targeted at minicomputers, and it seems, the PDP-11.
Here's a mention in IEEE Computer, about the new products at the 1976 NCC: https://www.computer.org/csdl/mags/co/1976/06/01647391.pdf (That was about the same time as "The Milpitas Monster") I can't find of photos of the thing. Has anyone run into one of these? --Chuck