The 'first one' found on the upper part of the Luger, is the serial number.
You should find the same number on the front side of the frame with a suffix (letter) below.
The last two numbers from the serial #, is stamped on various parts on the gun. The sideplate (as you refer to as 'second one') comes from a different gun, with a serial that ends with 73 (or is 73).
So the gun above is not matching, but a mix from different guns.
It can still be, and probably is a great Luger.. it's just something we collectors care about
