That's the "Portable Publisher", a video card allowing you to connect the Portable to an external display. Seems to have been made/distributed by Generation X Technologies.
So there should be a cable that connects to the header on your board so that monitors can be plugged in, presumably with a DB15 port.
The giveaway that it's a video card is the RAM on there, and the three different oscillators.
Here's an ad for it from MacUser June 1990: