Maybe this will help clear it up, some.
https://www.nhra.com/news/2021/421-n...orever-footage
The 421SD engine was available to some racers, to run in their '61 model cars.
“Pontiac is now offering to qualified drivers a 421 cubic inch high-performance engine option. The engine is rated at 373 horsepower and features dual four-barrel carburetors, a solid-lifter camshaft, and high-capacity aluminum exhaust manifolds. The 421 engine is available only with related heavy-duty driveline components. It can be fitted to any Catalina or Ventura 2-door model.”
NOTE: For MOST of the info presented in the Jr. Stock book, or any other drag racing book about stuff that happened back in the early days, I have absolutely no way to tell exactly what info is correct & what info is not correct. So, if I post something that was printed in the book, that was incorrect, I am NOT trying to deceive anyone. Example: The caption of one pic said a car had a 421 engine. IIRC, it was Mark Yacavone who knew the car & said it actually had a 389. I have no way of knowing what engine the car had. Hey, for all I know, the guy may have run both engines, at different times. Lots of drag cars have run different size engines.
Here is a pic of a '61 Packer Pontiac. Has 389 on the side. Could have a 421 under the hood, or not. The 2nd pic is of a '61 Pontiac which has 373hp wrote on it, & A/S on the windshield. I assume it was one of the '61's that got one of the over the counter 421 conversion engines. But, it could actually have had a 389, or the 373hp 421? Point is, writing on the car body doesn't always agree with what engine is actually under the hood. At least, I don't think there was ever a rule about that.
The pics I have of cars which have SS/S on 'em are of '62 models. Several '61's have OS/S on 'em. IIRC, that was a one year class. Pic #4 shows the A/SA car you mentioned. Don't know if it is of the final round you mentioned, or not. If so, looks like the loser was a '61 Pontiac ragtop, #620. Don't know if any of this will help with your fact finding, or not.