At long last I tried a little more brute force on it and got it apart, then tested each of the five cells individually; two are dead (milliVolts rather than 3.6V or so), so why did the charger indicate that it was charged? I'll try zapping each of those two cells with a 12V power source later, but I Googled the markings that I found on each cell -- Samsung INR18650-13Q -- and came across the following site, which indicates that the battery packs from many different brands use these same cells, although the effective price per cell varies considerably from brand to brand
(about 2/3 way down the page)
Just $9.80 per cell for the Ryobi vs. $17.80 per cell for Craftsman. (The posting is a couple years old, so prices could have changed.)
And note the post further down that points out that the supposedly 19.2V Craftsman batteries use the same five 3.6V cells.