Keg Post (liquid/out) ball lock
This post has is meant to fit a soda keg that uses "pepsi" or ball lock quick connects. Specifically this is meant for the Firestone brand of "pepsi" soda kegs. The poppet (inner spring piece) comes with this product.
Want to make sure you have the right part, check out this pdf.
For Firestone V & VI Challenger and Super Challenger Tanks
5/8 - 18 Thread Size