The photo directly below of a removed well seal, shows a rubber bung sandwiched between the 2 larger upper metal support plates and a smaller diameter metal plate. The 4 bolts with nuts on the topside, when tightened, cause the rubber bung to be squeezed between the upper and lower plates, causing the rubber bung to expand against the inner walls of the casing, thereby preventing easy removal unless the nuts are first loosened.
As already stated, full removal of the nuts will typically allow the bolts and lower plate to drop to the bottom of the well. By simply loosening the nuts without removing them, the rubber bung and lower plate may then be pulled-up or pried out from the casing with less risk of them also falling into the well along with the bolts.
The metal plug shown threaded into the OP's upper metal plate, is often utilized for installation of a screened vent to prevent insects/rodents from entering while allowing air to enter as the water level in the well becomes lower while being pumped out, and air to exit when the water level is rising during recovery.
The photo below, shows a well cap commonly utilized on wells equipped with a pitless adaptor.