I scrolled and had to scroll back for a longer second look all I can say is "DAYMNNNNNN" (southern). at LEAST a 69
Under-graded, just based on photos...very good but not in-hand analysis with other details not seen or clear. Still, looks 67-68 to me. I mean, if that's a 66, what else does it take for 67-68...again based on photos...?
You can't grade proof coins from photos, the hairlines can only be seen when swirled under a light. PCGS saw the coin in hand, and PR66 is an extremely high grade for a seated liberty coin, I will defer to them.
When I first looked at the photos, I assumed that I was looking at some kind of modern commemorative or something. Then after reading the slab label a couple of times, I realized what I was looking at! That aside, I'm an ancients collector, so I have no experience in distinguishing 66-67-etc. But, this coin is a good example of a TPG issue that I tend to see across the board. The issue is, other than the grade or categorized descriptions, the TPGs do not provide any information about how these things were determined. It would be nice if they would include notes, comments, or some kind of score card to explain how the decisions were made. An example in the world of ancients, I've sent coins in for grading that come back in body bags labeled "Tooled". Tooled can mean anything from a bit too much smoothing on the devices to complete re-sculpturing of the bust. How hard would it be to provide some indication of what exactly is meant? With a coin like the one in the OP, it would be really simple to provide notes such as "scratch on the U in United" or "Surface imperfection at 3 o'clock in the reverse field". Then we all wouldn't have to sit here and guess at what they were seeing!