That's probably an aftermarket accessory, but it looks to be a pretty practical thing to have. The missing spring could probably be replaced with a common hardware item, using one of the existing springs as a template for selecting the right length replacement, with or without putting the J-hooks on the ends. Does anyone else remember the "anti-clunk spring" that GM put inside the TH-350 transmission?
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!