First of all, I want to say that is some beautiful photography!
I agree with John S. that the odd side plate is the result of the refinishing of the entire gun. The process of refinishing was pretty well done as most of the contours of the piece have been preserved and not rounded off by over buffing. Also, the original markings have been preserved, even the Erfurt rework stamp for the hold open modification and the First Issue (sagely identified by Vlim
) proofs/acceptance on the left side of the receiver.
The side plate is unnumbered. It is possible that it was so severely pitted that the refinisher removed the number but judging from the contours I suspect that it is a heavily reworked unnumbered replacement. In addition of the thinning of the “bump” to the extent that the transfer bar is exposed, the whole thickness of the side plate is considerably thinner than stock…so much so that the upper left corner is square lacking the radius of a standard 1908 plate and the rear of the plate is almost flush with the frame so that the entire plate looks like it came from a M1900.
An interesting piece.
Ron