Bobpotter,
Welcome to the forum. Lugers were marked in series of 10,000 with a serial number and a small letter that is part of the serial number. Generally, the frame will have both the number and letter and the barrel will have just the number. However IAW Jan Still's third Reich Lugers x is not a listed letter for 1938; it should fall between b and n. 1937 and 1939 can have x's but not a 1938. Post a photo as suggested and maybe we can identify the letter. Regarding the Mengele connection, most likely someone is pulling your chain or trying to con you.
Tom
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