I have a 1978 Walther Interams PPK/s in .22LR with the black plastic thumb rest left grip panel and no banner on either side. I have seen another identical .22LR PPK/s, also a 1978 vintage, with the standard black plastic banner grips. This gun was serial numbered 252 higher than mine. What dictates what guns get what grips? Are the thumb rest grips a special order item?