I'm guessing here - just try screwing the elevation adjustment all the way up. It may just unscrew completely. There may be a light coil spring under the horizontal leg of the aperture leaf, holding it up against the elevation screw. Watch out for the spring-loaded detent pin.
Do you have the owner's manual? Take a look at the exploded view. That may give you a clue to how it's put together.
And for the record, I did have parts fall off mine - the roll pin & washer on the right end of the windage screw.