A serial number of 6172 and a barrel length of 4.75" makes it a Model 1900, and falls into the US Test Trials serial number range as MFC indicated. It is impossible, given what you have posted, that it is originally 9mm. The "GERMANY" marked grips would not be original to the gun, as US Test Lugers were not so marked. The magazines, as you have described them, are not correct for such a Luger.
I suspect that you do indeed have a mis-matched US Test Luger, but photos are absolutely essential to further identification. Photos of the markings, specifically the serial numbers externally and internally, the original grips (same deal...inside and outside), several overall views of the gun (left, right, top, bottom, barrel markings) and a few views of the holster are good starters. You may have a sleeper.
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If it's made after 1918...it's a reproduction
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