It is a reverse piston return spring, GM part no 3789061. GM once sold them in boxes of 17, enough to do one unit.
64-66 Olds Jetaway
?? Pontiac version of the ST-300 (don't have many parts books for Pontiacs)
62-73 Chevy Powerglide and Torqdrive
There's a couple of current auctions on eBay for incomplete boxes of springs. Looks like others needed just a couple, then put the rest on a shelf decades ago. Just Google GM 3789061 for even more leads. Luckily the item had high usage in Chevies.
What has been, can be again. (Bob Wills, 1942)