Came from older Chevy dealer.
No brand or numbers do I see.
Whatsit?
I will say I've never seen a device like this. But by removing the stamped rocker arm, looks like it could compress the valve spring by pulling on that long extendable handle. On the old Chevy V8.
Not a lifter tool. It looks like it releases an internal retainer.
Speculating it is a transmission shaft release tool....
It certainly resembles a tool used to compress tappet springs in a diesel fuel injection pump. The slotted part would connect to a bolt on the pump bottom, the pivoted piece would force the tappet against the spring (as you push on the handle) and you would insert a pin to hold the tappet in position. With the tappets "up" the pump camshaft can be moved into place.
Like I said it closely resembles such a tool - not sure if that's what we have here. Al.