AFAIK, this is a male Stigmatomma pallipes. I found several of these males sitting on understory leaves in a fairly open forest. The guide's genus page says that there are 3 species of Stigmatomma in "our area" of which two are shown in the guide; S. pallipes and S. oregonense. The latter is out of range according to Antwiki. According to AntWiki, the 3rd U.S. species is S. trigonignathum, known only from 2 individuals collected in North Carolina. Somehow, I don't think I have to worry about that species for this ID :-)

There doesn't seem to be much of a consensus in the literature on the proper bionomial name of this ant. ITIS doesn't recognize the genus Stigmatomma at all, instead retaining Amblyopone. Nearctica doesn't include any of the above species names, instead listing S. serratum, S. subterranea, and S. wheeleri each in the same entry as Amblyopone pallipes.

I'd appreciate confirmation/correction of the ID. Thanks for the help.