I've come across something I cannot identify in numerous American foundry books even though it is quite unusual. A metal caps-only font which I suspect is late 19th or early 20th century. No pin mark or other identifying marks. It has the usual nick and groove, although the groove is off-center and seems to be deeper than normal.

Appears to have been called Fifth Avenue Monograms (ATF), according to this post on the WhatTheFont forum.

Another source.

And it has been digitized.

@Kevin Thompson Thank you! It never occurred to me to look at monograms. The closest thing I finally found late last night was Head Letter No. 2 from Keystone. It appears to me the ATF monogram type was closely based on that design.