I also think it is related to the reloading of shotgun shells (or maybe their initial manufacture). No markings or other clues? No other pieces of the puzzle?
I am not so sure that the manual belongs to the device.
Here is what the person who snapped the pics said:
Found at the gun show.
"I overheard the Lt. Col. WW2 veteran/military collector who was
selling said he would pay someone if thfey could tell him what it was.
It came from a military junk collectors gun vault when he passed away.
I was curious so I snapped those pics."
and more info from the picture taker yesterday:
"MAE 213 is what's scribed on the side of it I think.."