Sorry, I didn't mention in my original post that, although I'm often a Lightroom CC user, I do send all of my non-Lr iPhone images to Photos ... and, that I did migrate from iPhoto to Photos years ago.
However, I did find some more recent photos that I'd mistakenly placed in iPhoto but not in Photos, not very clever of me. In preparation for upgrading to Catalina I discovered (not surprisingly) that iPhoto was 32-bit. I first checked to confirm that iPhoto was truly 32-bit and came across your answer to ZzyzxOh back on August 30, 2013: "The most recent version of iPhoto is 64 bit." Therefore, I decided to ask my question today. Btw, I don't know if you can remove that answer or update it. Doing that would probably be a good idea for other Mac users.
Your reminder of how to make certain that all of my iPhoto images have been migrated to Photos was REALLY helpful.