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 OEM band adjust tools and proceedures. But, it is possible to perform an on-car band adjustment that have external adjustment screws.
During a rebuild, the bands are preadjusted to OEM spec.
From "Fatsco" On-Car Band Adjustment PDF Click Here
From "Auto Trans" On-Car Band Adjustment Click Here
From PontaicSafari.com Pontiac On Car Maintenance Click Here
From Old Car Manuals site Click Here