Here’s the answer I got on another forum when I was doing research on my Pistole 640(b) (German Hi-power). Look at the Waffenamt stamp on your pistol and you’ll have your answer. I did regret selling it but I needed some cash to finance another firearm purchase. Good luck
''WaA 613 = The very first Hi-Powers with the slotted shoulder stock cut and the tangent rear sight will be marked with the Waffenamt stamp WaA 613 and were manufactured from from May 1940 until early in 1941 with a serial number range of 44,500 through 65,200.
WaA 103 = The next group of Hi-Powers were made from January of 1941 until May of 1942 and will be marked with the Waffenamt stamp WaA 103. Some reports claim that the WaA 103 marked Hi-Powers were only made during 1941 so the research is still ongoing. These pistols will be found with the tangent rear sight but no shoulder stock slot. They fall into the serial number range of 65,200 to 95,000.
WaA 140 = The third and final German WWII era made Hi-Powers will be found with the Waffenamt stamp WaA 140. These WaA 140 stamped pistols were manufactured from late 1941 until the liberation of the FN plant in Belgium in 1944. None of these pistols will have the shoulder stock slot, but the early made examples in the serial number range of 95,000 to 135,000 will have the tangent rear sight. When the tangent rear sight was removed, the Germans replaced it with a fixed rear sight that was drift adjustable for windage only. It appears that the first of these WaA 140 marked fixed rear sight pistols begin at around the serial number of 150,000 which leaves a gap in the serial number range between the tangent and fixed sight WaA 140 marked pistols. The fixed sight WaA
140 marked pistols continued until around serial number 200,000 which dates them toward the end of 1942. At the beginning of 1943, a new serial code numbering system is used which began at number 01a until number 99999a was reached. Then at the beginning of 1944 the serial number 01b began and continues to about 6300b, at which time German production ceased due to the liberation of the FN factory. All of the letter suffixed serial numbered pistols will be found with the fixed rear sight.''
Hope this helps