They are lateral MOSFETs and the case is the source (unlike normal MOSFETs where the case is Drain).Interestingly, the topology in Jony130's attached file, 88_1235904466..jpg , was used for many years by QSC in their commercial amplifiers. The key advantage was that the collectors of the output transistors were all at "ground" potential, and could therefore be used without mica (or other) insultation on a common heatsink - which was also grounded. Substantially better heat conduction without the insulators...
It can also be done with the conventional circuit, but the heatsink is connected to the output. It means you can't connect the heatsink to the case, but generally, you can't anyway because it would exceed the standards for case temperature.