Mobuis- That's the 3rd body gasket. Swiftech now uses 3 body gaskets for their TEC assemblies instead of 2.
First you install the 1/8" thin gasket, then the 226W pelt and a 3/8" thick gasket and the cold plate and finally another 3/8" thick gasket. That's it.
FYI the 3rd, thin gasket also offers extra support for the peltier's solder joints, which are very fragile.
Bruce