Before the Vokes filter they improvised...There are various photos of different 'field fitted' air filters that aren't in the parts lists...
No doubt they were sourced from vehicles, generators or anything else that had them fitted...
One North Africa veteran I spoke to said the engines in his bike only lasted a few weeks without a filter and running on a mixture of ingested sand and very fine dust particles...Ian
Had known a few mechanics ex Malasya and PNG.
The battle roads and a lot of the tempororay landing fields were made from crushed coral.
The supplied filters would choke within the first hour of use .
Without the filters the coral would deposit on the piston crown till it hit the head & bent the rod.
Apparently the most popular "field filter" was a vertical screen with several socks over the top of it so you just pulled the top sock off whenever the engine started running too rich.
Army being army we supplied thick woolen socks to the troops which do not suit the tropics so boots on bare feet was quite common.