What is this tank in my W123?

[timd]

I have recently acquired a 1982 w123 200T and have been poking about to see what I've got. There is a tank between the spare wheel and the outer skin with three fuel type hoses connected to it. What is it?

Tim
1982 200T

 

[guydewdney]

fuel evap canister?

vacuum system reservoir?

 

[TimN]

It has someting to do with the breather for the fuel tank but I have never had the need to look inside.