Sherman tank machine gun sight?

[Jholl72]

Hello,
I have been a member here for awhile but this is my first post. I picked up this item today and am not sure what it is exactly. I was told it was a WWII Sherman tank machine gun sight. I have no idea if that is true or not. There are four layers of glass inside the steel case with one black and one white aiming cross.
The only marks on the item are on an inner piece of glass:
OUTWARD and DSG.

Can anyone please positively identify this thing?