Welcome to the forum. That's a very nice Luger and one you can be proud to own. It appears to be in excellent condition.
The over stamp on the frame serial number may be simply to correct a mistake. The font looks to be correct and of the proper size. But, at this late date that's only a guess.
The "yellow" in the safety marking was originally white and has simply aged into a yellow tint.
The $600 you paid would be considered theft in the States. It would bring three times that here with the holster. Congratulations on a truly great purchase.
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