This page offers some helpful info about Early Hydramatic On Car service and band adjustment. In most cases it is better to use the prefered band adjust tools and proceedures. But, it is possible to perform an on-car band adjustment that have external adjustment screws.
From "Fatsco" On-Car Band Adjustment PDF Click Here
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From PontaicSafari.com Pontiac On Car Maintenance Click Here
From Old Car Manuals site Click Here