Thank you all for the replies!
I don't see any evidence of damage to the surface in this area, new or old. The sharply angled formation is raised, and there are no gouges adjacent to the raised area. I've viewed it at 100x, and the feature does not have the look of a damaged area. That said, I'm not sold on this being a horizontal D.
By the way, while only a G-4, the angled formation is pretty sharp, maybe too sharp, and I'm not
quite sure this coin is entirely ready for the copper recycling station in the sky. The deadly combo of overworked dies, poor quality control, metal impurities in the planchets, weak strikes, and 101 years of use and abuse make anything circulated a scavenger hunt for early Denver Lincolns. (You should
see some of the shrapnel in my 1916-D junk box. This one looks mighty fine compared to
some of the scrap metal in that box!)
That said, there seem to be three possibilities for this coin.
1. It
could be a D/D/Horizontal D. (There is a minor D/D East, but it isn't as clear as WRPM-001, and something this minor doesn't terribly interest me.) The big question is whether the angled formation N,NW, and W of the D is the remnant of a horizontal D. THAT would interest me a lot. I'm not convinced that's what I am seeing here, but I'd still like to rule it out with a little bit of confidence before tossing this critter back into the box.
2. It
could be the result of die damage. That wouldn't surprise me. Quality control was a *bit* lacking in Denver in this era. While mildly interesting, die damage isn't something I want to collect, and back to the scrap box it would go.
3. It could also be a planchet defect. There are a lot of coins from this era with obvious impurities in the metal. (The so-called "straw" coins are an example.) This is what I'm leaning toward it being, rather than a horizontal D remnant. If so, the defect simply was in the right place at the right time to be near the mint mark. Here's another 1916-D with a 90 degree angle metal impurity evident in the field NE of the mint mark, halfway between the D and the 6.

Given the new pic, any additional thoughts?
Again, thank you very much for the feedback. Discussions like this are an enjoyable part of why we all collect coins.