Embodiments of well plunger systems, assemblies, and apparatuses are described. In an embodiment, the apparatus includes a body having a first open end, a second open end, and a channel extending from the first open end through the body to the second open end, the channel for passing fluid from an oil or gas well. The apparatus may also include a receiver disposed at the first open end, the receiver configured to receive a stopper configured to at least partially seal off the channel, the first open end comprising a retention member receiver configured to receive one or more specially adapted retention members for applying a retention force to the stopper when the stopper is engaged with the receiver.