This sounds like the typical machining mark for Lugers which have had the sideplate "forced matched" to the rest of the gun.
If you examine your sideplate, you may find that the area to the right of this ridge is shallower, or lower than the balance of the sideplate. By removing all this material, they are able to obliterate the original sideplate serial number without simply "dishing out" the area of the original serial number and thus providing an even, smooth surface to re stamp.
They did the machining in this fashion to avoid contact with the raised trigger bar recess on the sideplate.
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