We know that the Germans used some types of filler in the late war P.38 bakelite grips to conserve the base material.  These grips show more filler than any I have seen.  The inside of the left grips shows an amazing amount.  Last, there are two super closeups of the inside of the left grip where you can see the fiber in, what appears to be, shredded light cardboard or very heavy paper.  It also looks like there could be some pieces of coarse fabric included.  It's interesting how relatively consistent the outsides of the grips are and yet the inside of the left grip has so much more filler showing than the right. Since these have the 1528/1529 markings on the inside and are so obviously late war, I assume they were made for a Walther P.38.