Chip,
I would advise that you abandon your quest for matching parts. According to Jan Still's serial block information there would only have been about eleven 1918 DWM receivers marked 4757, and only six WWI DWM barrels marked 4757c. Proof marks on the receiver would also flag a force-matched receiver with a "matching" number. Even matching lugers are infrequently encountered with matching magazines. Consequently there is no point in seeking out a matching sideplate, and there are other indicaters besides the external numbers on a sideplate that reveal whether it is original to the gun. Enjoy this gun for what it is.
A correct front sight is probably available from Tom Heller (Lugerdoc) on this forum. However, most Lugers shoot high at normal distances and a taller P-38 sight will fit and provide something closer to point of aim.
KFS
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