You should have these proofs if it is 100% German:
Since it is a 40SW and you said you could not find any proof marks, it could be one of the 40's Smith & Wesson made the slide and barrel for. If this is the case, the frame would also lack the "eagle over n" proof mark, indicated in the location below.
This consists of a "eagle over n", a two letter date code, and then a stag horn.
The codes decode as such:
KG = 1996
KH = 1997
KI = 1998
KK = 1999
AA = 2000
AB = 2001
AC = 2002
AD = 2003
AE = 2004
AF = 2005
AG = 2006
AH = 2007
According to
Mr Lunde's Fact Page
"If the slide, barrel, and frame are lacking the "eagle over N" proof mark, that's the first indication that it is made by Smith & Wesson. The frame was still made in Germany by Walther, but the slide and barrel are made by Smith & Wesson. Only some .40 Auto P99s were made by Smith & Wesson. All 9mm ones are 100% German-made. Another indication is if the second digit of its serial number is 6 (meaning that the serial number follows the pattern "46xxxx")."