It was standard procedure to remove the sight bridge when finished shooting, because there is a great danger of bending the sight if it is left on the rifle.
There is a knob on the front of the sight base that can be unscrewed a few turns, and then pushed in, to release the sight bridge that can be pulled up and off.
The sight has vernier marks. There are two ways for a shooter to zero the sight. At 300 yards, set the vernier to the 300 yard mark. Then set the sight to whatever yardage you are , to be close.
Or (and this is how most shooters do it) set the vernier to 10 minutes when zeroed for 300 yards.
Until you shoot it, you don't know how it is zeroed. It might be set for the last distance shot, say 800 yards, so current setting might not mean much.
If you can shoot it at 100 yards, try a setting of 0 wind and 10 minutes of elevation. That should be on paper.