plastic grips for luger

Can anyone help me about the original plastic grips which Mauser used on their late production P08's. I have just purchased a 1939 S/42 with reddish brown plastic grips with a cirle moulded into the grip. They are chequered and in excellent condition. Are these original?. The pistol itself is all matching and in excellent condition.

any info would be greatly appreciated.

Fred