UBB.threads Forums Edged Weapons The BCN Bayonet Forum Horster K98 Grip plates 38-41

Chaps,
I know the recognised grip plates on Horster K98's between 1938-41 are almost black but occasionally I have seen them with a mottled appearance and I have recently picked one up ( matching 2106m 1940 ). I found the picture below in the archives of another one dated 38 and wondered what the general consensus is on the cause. Is it purely down to a poor mix in the grip material and is it unique just to Horster production or across the board?.
Thanks for looking.