I have used one of these long ago. I used it as an adjustable picatinny alignment tool, or a way to centre punch the screw holes on the picatinny to align with a barrel. There should have been a 'card' device to attach to the muzzle that came with it, identical to the ones that come with a laser level.I need to know the exact name of this tool.
Laser for picatinny.
Thank you for your help
First, you can attach it to something like this (random internet photo) and then adjust the screws on both sides of the rails to align with the barrel.

You can also take a barrelled action and use it to help align custom holes in the action, to ensure that a bore laser (or eye-ball'n it) and this laser align with the center line on the action. If they don't it could mean there is a bent barrel or a not-square thread/attachment into the action. It's good for the 'quick verifying check' to other more accurate tools and methods.
At least that is the way I used it.