OK, I finally got it. Upon inspection, the finish is very likely original, as it is not nearly as nice as the somewhat dark original sellers pictures indicated. Edges are crisp, machine marks evident, the inside is white...rampant oxidation and freckling all over...I don't see any reason to suspect the finish.
The interesting proof on the rear looks like a double stamp, though I haven't found an online image of what its supposed to look like, it looks like a poor stamp offset a few MM's, and re-stamped.
It is still there, which is the interesting thing.
The side plate is still questionable, though there is a second matching serial number on the underside. This may be the clincher, one way or the other. Were these side plates ever double marked? Side and bottom? I've only seen one or the other, but I'm not as experienced, nor have as many books as many of you. For what it's worth the stamp on the underside matches the stamp on the safety bar. There are nothing but machining marks on the inside of the side plate.
Everything else matches, grips, striker, ejector. I stayed up late last night taking pictures and putting them on my web page, though I haven't added much commentary yet.
I ended up getting it for much less than the original asking price, so I am happy with it, even if it turns out the side plate isn't correct. Other than the odd font on the side, and the double location stamping, it APPEARS correct, meaning the hue, oxidation, scratching, etc. all balance with the rest of the pistol.
I have found one defect with it, the "L" shaped spring inside the takedown lever shaft is broken and missing half of it's original length. Enough is present to function, but it shows up as a "hole" when looking at it from the side. I don't know if I am going to do anything about it or not. It's not a serial numbered part, but as I don't really intend to shoot it, it's not a pressing issue. So I'll probably just leave it as is for now.
http://home.comcast.net/~gunspotz/dwm1916.htm