Some further web searches on my part turned up a brochure for the I/O Pak Computer/Typewriter Mechanical Interface by Rochester Data Incorporated:
The Rochester Data I/O Pak is an electro-mechanical device designed to interface between a conventional powered carriage return electric typewriter and any digital computer configured with a suitable electronic interface. The unit consists of a bank of specially designed solenoids mounted in an array that fits directly over the keyboard of the typewriter. Energizing a specific solenoid causes the respective typewriter key to be depressed, thus printing a character. Electrical actuation of the solenoid is accomplished by self contained drive electronics that operatein response to the selection of a pair of one-out-of-eight control lines.
The two models of the unit will operate virtually any electric typewriter that has powered function keys (carriage return, backspace, etc.) and a U.S. keyboard, with no mechanical modifications to the typewriter. All adjustments are self contained in the I/O Pak itself.
The unit features low profile, easy initial installation, instant detachability and replacement, modest power consumption and high reliability.
The brochure goes on to say that it is compatible with "all commercially available, powered carriage return typewriters". It has a general-purpose interface that is compatible with "almost any computer", as well as model-specific interfaces for the TRS-80 and Apple. A Commodore PET and industry-standard Centronics interface were under development. The company offered configuration software for the TRS-80 and Apple, plus a patched version of the Electric Pencil word processor. The entire kit (solenoid drive + computer interface + power supply) was available for just under $1000.
I couldn't find any videos of the I/O Pak in operation, but I did find this YouTube video of a homebrew device that operates in a similar manner: