Oil filler tube, part missing ?

[GM6VXB]

Been trying to get my head round a minor but iritating problem I have with the oil filler tube.
Suspicion is a part (filter ?) missing, but not sure.

Basically when the engine has run for any time, when I take the oil filler cap off there is always
a small amount of oil near the top of the filler tube. No leaks so suspicion is oil splashing up the tube
and setting in the lip near the top of the tube..

Looking at the parts book the filler tube is shown as BA128 complete with parts BA129 and 130.
Now BA 130 is the filler cap, so I am guessing that BA129 is some sort of filter ?.

I know at some time in the past I did see some reference to this but more pointing to filling with oil
rather than stopping oil splashing up the pipe.

Anyone know what the 'missing' part is and how to make/buy one ?.

Thanks,

Martin, GM6VXB