Well, once again my ignorance is broadcast to the universe. Went to the Mercruiser site and saw familiar GM engines all over the place: 4.3 V-6s, 350 and 454. I guess "Mer" is just French for sea, and it doesn't have to be anything about Mercury. Lastly, I had asked about what GM truck engines came before the 216 ... then I found Barry Weeks' tech tip note on the 194 and 207 of the '20s and '30s, before the 216 came along in '37. Thanks, folks, for the info.
