Need help with Luger markings

[mesalyers]

My dad was given a Luger by a friend of his and there are some markings I can't find a reference to. I'd really like to know the history of the gun. The S/42 and Crown N tell me it's a commercial model made by Mauser (right?) but the serial number has been x'd out and restamped. The last two digits on the old AND new serial number match the numbers on the rest of the gun. What does the restamping mean (who did it and why?) Also, what does the scripted letter "u" refer to in the front? Finally, it's not for sale but does anybody have an idea of it's worth? Thanks in advance.