Might as well see if it'll wake up, first. Spent all of ten minutes soaking the moving parts with very light oil and working the mechanisms and it freed right up. There are still some issues -- one letter's
After getting it going, I took a long look at the Mysterious Inner Workings. The typical ribbon advance and reversing mechanism on small portables uses a sliding linkage to move right-angle gears into (or out of) mesh at the ribbon spools. So (generally) there's a long, transverse rod or shaft that moves a short distance back and forth in the same direction as the carriage and it gets rotational drive from the same setup that advances the carriage when a key is struck. See the rod and tiny gears?
It's pretty obvious on this Underwood but drat! It wouldn't slide back and forth. More stuck stuff? Bumped it and something went click....Oh!See what it does? Rotational drive is at the center, a little hand like on a revolver that advances one tooth per letter and the shaft is pivoted at that point! To change ribbon feed direction, the shaft pivots away from one gear and into mesh with the one on the other side. Pretty tricky! The 90-degree drive is a worm type, with a helical "tooth" on the shaft, rather than the more usual flat-cone method.
Word to the wise: Don't play games involving mental agility for money against retired Underwood design engineers.