I've told this story before, but when I was growing up a friend of mine's Dad owned a Pontiac/ Cadillac dealership, so my buddy got a brand new car every other year. In 63 he got a 421 Tripower Gran Prix and in 65 he drove up my driveway with a brand new 65 GTO convertible.
The car was beautiful, black with white top and white interior, and every option you could get on a GTO. It was a tripower, four speed, and had 4.33 gears. At the track he was turning low 12's on cheater slicks, when most Goats were running in the low 13's. It was a monster, and he made a lot of money with it on the street.
One time he confided in me and showed me the window buiild sheet. On the build sheet that came with my GTO is said something like " Tripower 389, etc etc", but in that area on his build sheet was a big X across that whole area. He then told me he had ordered it with a 421 HO tripower engine. Outwardly it looked no different from a 389.
I've often wondered where that car ended up, because Pontiac swears they never built a GTO with a 421, but I rode in one lots of times.
Don