Either bouncing around in my backpack knocked something back into place, or simply popping the back off jarred things enough, but before I unscrewed the core to start messing with the keys on my old calculator I decided to try it again, and now the 7 is working. And nothing else broke in the process. So looks like there's life yet in the ol' 4012.
(Unrelated aside: comics tomorrow. UPS being shut down Thurs-Sun apparently delayed things.)
(Unrelated aside: comics tomorrow. UPS being shut down Thurs-Sun apparently delayed things.)