Bumper Filler

What is the purpose of the wooden bumper filler on the M38 and why was it not used on the m38a1?

That was a carryover from the MB/GPW, intended to strengthen the pitifully light gauge steel used for the bumper. By the time the A1 came along, the bumper was thicker and I suppose Willys dropped it as not really needed.

Bumper re-enforcement.
And (Provides) a firmer guide for the starting handle.