This is a coolant thermostat / thermostat housing, mainly for the M271 CGI petrol engine.
Common applications include:
C 180 CGI
W204
M271
C 200 / C 200 CGI
W204
M271
C 250 CGI
W204-M271.860
E 200 CGI
W212-M271
E 250 CGI
W212-M271
E 260 CGI
W212-M271
SLK 200
R172-M271
SLK 250
R172-M271.861