I first queried The Mudcat Café about this piece of music in August 1999. The fact that I had been unable since then to elicit a credible lead [see the final paragraph in my blog here, the Google+ posts here and (a reprise of the blog item) here] has been a thorn in my side! This evening I was going through some random old-time favourites in my music library and I thought again to look for it. Google showed me the Mudcat link and I went there to see how long ago I had posted my question. As I advanced the thread I reached a January 2017 guest posting with the above YouTube link.
To be fair, Anderson's voice wasn't exactly how my mind had recreated the memory of it, but there was little doubt that this was the song that I had last heard on the radio circa 1970 — almost fifty years ago. The YouTube audio had been up since September 2013. Three-plus years for someone to link it to the Mudcat thread and two more years for me to notice it.
Over the years I had convinced myself that I would never know the artist/song before I died. But my faith in the internet has been somewhat restored and I can cross this difficult things-to-find task off my bucket list.