Never mind: in spite of my searching like a crazy, the "Similar Threads" box below rubbed my nose into this post: https://www.mobileread.com/forums/sho...d.php?t=164473
So I tried converting the offending books from PRC (their original format) into mobi and re-uploading them into the kindle.
It works. Exactly as I wanted.
Sorry, I feel something of an utter idiot
But really, there's no blatant and visible way for anyone using Calibre to see if a book already imported has DRM or not in it. Or there is?
And thanks to Mr. "Deep Cover"