If you look carefully, and use calipers to measure the areas, you'll see the slight grind marks and bevel in the areas of the frame and receiver where numbers were removed. The numbering dies used by Mauser on "G" date pistols I've seen have a different typeface.
The "X" looks like a multi-strike on the front of the frame, an unusual place for it.
This could well have been a training piece used when helping educate the gunsmiths that reworked your pistol.
Separately, you might want a copy of our FAQ document.
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http://forum.lugerforum.com/faq.php
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