You have an interesting user name. Welcome to the forum. I have been studying Mauser pocket and vest pocket pistols for some years now. The WTP II is most often encountered with black grips. Occasionally one will be encountered with brown grips. I have 8 WTP II and only one has very dark brown grips. I have seen some that are lighter brown, a medium brown. As far as I can see from the 100s of examples I have recorded in my serial number database there is no rime or reason to why or when some WTP IIs have brown grips.
Regards,