Was in the pile of metal I bought at auction yesterday.  It’s about a foot long, 1-1/2 thick and 3 inches wide.  When you pull the “trigger” a section on the inside pulls back about about. Quarter of an inch - obviously just to hold something.  

It's a thing-holding thing.

It's a shifter handle off some old piece of machinery.  The little latch kept it in place so you wouldn't have to worry about vibration causing it to skip on you.

Probably engaged a "sawtooh" edge to  act as a ratchet to hold it in place.