Can somebody please explain the antenna and mixer stage of this AM superheterodyne receiver?
If this is a really old-style receiver, it most likely has vane-type tuning capacitor for selecting the receivefrequency. These are notorious for having intermittent shorts between the movable and stationary vanes.

Look at the value of the capacitors in question. If they are large value, the capacitors are most likely to be simple coupling caps intended to stop damage from occurring should the tuning capacitor develop a shot.