The oil bath air filter was fitted to a lot of “Colonial” Slough Tractions. It was fitted to some, if not all supplies to Australia, South, East and Central Africa and probably elsewhere, where conditions were dusty.
My South African L15 is fitted with one. It is bulky and you have to be careful to avoid it rattling on the bonnet. However, it does have the advantage of not requiring a filter element. The parts lists do not show this filter but do show the French equivalent as well as the cartridge units. I am sure that you will have no trouble finding one. Have you tried Chris at spares?
Roger