Thanks Kirk and Husker.
Here are two more shots of the new mirror, along with one of the old style at the bottom (that's the passenger side, sealed in a bag from the prior owner). The new ones are from PI Motorsports. I don't know where the old ones came from--the mirror vibrates inside the housing, even at idle, so other cars are just a blur.
Kirk, do you think with just 3 wires I have either horizontal or vertical movement plus a ground?