FELGE CJ2/CJ3/M38

All Willys MB jeeps built after vehicle serial number MB120700 and all Ford GPW's were equipped with 2-piece split-type "combat" rims.  Combat wheels are easy to spot by the eight bolts holding the two halves together.  

Very early Willys MBs (the so-called Slat Grill MBs built in late 1941 and in January 1942 through serial number MB120700) were equipped with a standard 4 inch drop-center "solid disc" wheel.  All WWII jeeps used 16 inch diameter wheels.  The standard tire was a 6.00X16 NDT (non-directional tread). 

This picture shows a 4 inch wide solid disc rim (left, gray) and a combat rim (right, olive drab) side by side.  The solid disc rim got it's name because it's center part lacked the vent slots we typically see in other automotive rims.  The heads of the eight bolts that hold the halves of the combat wheel together can be seen.

Solid disc wheels of this type are found in two widths, 4 inch and 4.5 inch.  The 4 inch wide solid disc wheels  were supplied on prototype jeeps in 1941 and on the early MB's as mentioned above.  The 4.5 in wide solid disc wheels were supplied with early civilian jeeps just after WWII  (cj2 and early cj2a jeeps) and also on a few "specified" MB's that were equipped with 6.50X16 tires through serial number MB120700, but there are differences...  

This image below shows a 4 inch and a 4.5 inch solid disc wheel side-by-side.  From this vantage point they are nearly identical...