Now I could identify the letter behind the serial number (it's better to be seen on the picture of the left front area), it's "e".
So the complete serial number is 8242 e.
According to the 4th German edition of the excellent book about Walther pistols "Walther Verteidigungspistolen, Modell 1 bis PPX", written by Dieter H. Marschall, your P38 belongs to the 2nd execution of the P38 with code ac 41 (built 3/1941 until 11/1941), serial number range 4834b to 4530i, produced number about 70 000 pieces. AC is the code for the Walther factory in Zella Mehlis.
41 is the year of production.
The marks with a number below an eagle are military acceptance stamps.