The device is used to true the reticle.

What this device assumes is that the bolt and the barrel are centered along an axis drawn through the bore center.

You center the top pointer over the barrel and the bottom pointer is centered over the bolt center.

The V-block machined into the tool centers the tool on the scope.

You then look through the reticle slot and align your reticle to it.

The Chosin Few November to December 1950, Korea.
I'm not one of the Chosin Few but no more remarkable group of Americans ever existed.