There are 5 types of boiler used on the Faema E61 espresso machine. The most well known is the double flanged model. You have on one side the heating element flange and on the other side the water level flange, both are out of production. These are held in place with aluminium boiler rings. For the single groups you have a boiler with a flange on one side.
The next model is a stainless steel boiler, with one flange. This type of boiler is less common then the copper boiler above. There is also a later model stainless steel, which doesn’t have a removable flange. Both these boilers have corrosion issues, this has to do because of the stainless steel.
The latest model is also a copper boiler, this boiler has a single heat exchanger(HX) in the middle. Most modern espresso machines have for each individual brewing group a individual HX.