Yes, that's correct. Later transmissions like the aluminum case Powerglide (1962-up) and the Turbo 350 did away with the 2-piece adapter because there were no bellhousing bolts behind the flywheel to contend with. Then runout became a non-issue. The Dual Range Hydra-Matic was an incredibly precisely machined piece of equipment which was very expensive to produce. That's one reason Rolls-Royce used a slightly modified version of it in their cars under license from GM.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!