Alistair, Great clear pictures. Thank you! It seems you are correct , about the long one going in the outer hole.
Your fitting looks different to the one I have. (unfortunately mine is about as far away as you are from Italy) My one steps down in dia. and stays that way.
What is the part number on yours please?
I have checked in the later parts book (from Michael R)
(it gets tricky because the "engine oil cooler outlet" is the "oil cooler inlet" and vice verca, if you follow?)
The long one is CTL 7142 oil cooler inlet. It later became GAE 8741
The short one is CTL 7143.oil cooler outlet. It later became GAE 8739
I have not yet found any text confirming which fitting goes in which hole.
There is a picture showing the layout of all the cooler fittings valve etc. that someone might like to post for us.
My gut feeling is that the restrictor is all about feeding the mains . / big ends, while the cooler is filling.
I have yet to logic this out, to my own satisfaction.
Here is a picture from an un-recalled source.