The above info pretty well covers the gun.
The Germans used Bakelite for the grips.
This is made of ground up wood chips mixed with a polymer resin.
Both Reddish-Brown and Black grips were used during the war, especially later in the war.
The holster looks like a typical European-type full flap military holster that someone cut the full flap down on.
I have seen similar holsters owned by vets, who wanted a more open holster to carry their war trophy in, so they just took a knife and cut the flap off or down on a suitable holster.
It's possible that particular holster was originally for a Model P-35 Browning Hi-Power pistol.
WWII P-38 pistols, ESPECIALLY veteran's war trophy's bring some surprisingly high prices these days.
A look on the internet gun auction and sales sites show non-recent import P-38's going from $400.00 to over $800.00, and even more for some special versions.