What kind of gasket or seal ring supposed to be on the oil cap?

I just finished replacing 11 of the 16 valve clearance adjusting lifters on my 2006 E12 1.6L gasoline sedan and noticed that the valve cover is quite dirty around the oil filler cap. I checked the oil cap to see what kind of seal or reing should I get as a replacement from a local auto parts store but to my surprise I don't see anything on it that would resemble me to a seal or gasket.

I purchased this car as a used one 12 years ago so, I don't know if there supposed to be a seal on the cap and the previous owner lost it
this cap is such a weird animal that it supposed to seal by itself and if it gets worn out, you just have to replace it with a new one.

So, what's the story here?