This one looks like it might have flipped up, can't say for sure though. The most common type of lollipop sight simply screws into a deep, tapped hole in the tang, that extends an inch or more into the wrist of the stock. This will have very fine threads (64 threads per inch, for example) and serves as the elevation adjustment. It may, or may not, have a dovetailed slot across it with a bar containing the actual aperture, which can be adjusted for windage. There are countless variations of these sights, since most were hand-made by the gunmaker, or another gunsmith at a later date. I have quite a few original target rifles, some with tang sights, but more with empty holes where they used to be. I could send you pictures of some of mine, if you will send me pictures of the empty holes where they were mounted.
Curt