A rare charm for the 70s, almost always without mounting hardware. Here's one way to install and secure it.

Since in the trunk there was nothing to mount it on, I had to think outside the box. Made a plate from stainless steel (two holes and a bolt) and screwed it to the bumper mounting bolts.
Attached the pocket, marked where to drill the hole for the bolt.
Placed the pocket back into position, put it on the bolt
Secured with an aesthetically pleasing decorative nut. If needed, it can be easily unscrewed, allowing removal of the pocket and access to the jack.

That's how it's done!