Because the air cleaners on early jeeps from 1941 through 1953 are functionally interchangeable and easily removed and swapped, early jeeps are often missing their original, correct air cleaners.  Four styles of air cleaners were used during WWII jeep production...

1- AC "pancake type" introduced with the production start in late 1941. Instructions placed on a small decal (black letters on red background).  Those used on GPWs are the only air cleaners that are “F” marked.

2- Oakes "decal type" introduced in early 1942. Instructions placed on a large decal (silver letters on black background).

3- Oakes "plate type" introduced in mid 1942. Instructions embossed onto a steel plate which was attached by spot-welds .
 

4- Oakes "standard type" introduced in late 1942. Instructions directly stamped in raised letters on the air cleaner case itself.