This was a New Old Stock (NOS) machined tube with a pinhole at one end and a lens secured with a snug press-fit collar at the other.
It was 2-1/4 inch long and about 5/8 inch diameter. It is for focusing the light in the gun to a point for the targetting in DUCK HUNT, game #558.
Now sold out, unlucky to get more...I have dimensioned it so if you are handy with a small metal lathe you can make your own. You will have to figure out the lens though! See last photo...