I believe they just couldn't identify the mint mark. I'm not a vam guy though, maybe some of those hardcore lovers could find it or maybe the vam evidence was worn off but not knowing the mint is what I believe to be the issue. This was a very good example though for people who think P-01s can be made at will.
It can get p01 if they can identify the mint mark. If not they won't slab it. That's why p01 Morgans are so tough and collectible. There's a finite line where a coin is a p01 level of wear to a coin who's MM cannot be confirmed due to a little bit too much wear
So, then, are P-mint Morgans much harder to land in P01? I can imagine a level of wear that would leave the top of an O or S (or certainly CC) distinguishable, but leave a blank space ambiguous -- a grader couldn't be sure whether the coin had no mintmark, or had a lower-position one that was worn away.
I don't know for a fact, but your reasoning certainly rings true. There would have to be an area which is unambiguously "field" and not "wear."
Well here is a thought. I got curious one time in my lowballing days about the predictability of grade based on the weight of a coin. There is a definite correlation between grade and weight since the coin loses mass as it wears. That was years back and I wish I still had the stats. However, I don't remember the weight getting this low, so the note of caution from them might just be justified. For the weight to get this low the coin would have to start on the low end of weight in MS and then lose just about all possible mass along the way.
But that mass lost is marginal until you get to the G/VG grades. Plus how would you account for fluctuations in weights (yet still within tolerance)?
I actually weighed every coin I purchased and found the normal fluctuation of MS coins, which was slight because of planchet adjustment before coining. But there was a little fluctuation. You are right in that the mass loss was greatest below let's say VG, but then it started to show. The coin in question here was guessed to be either a FR-02 or PR-01 and there was enough loss there to be recognizable using a jeweler's scale. But I do not remember the loss being as great as what was shown on the pictures at any grade. A long time back and I wish I still had the data because it became interesting.
I did all raw Morgan Dollar purchases for a period of two years, so probably 200 - 300 coins. I believe it was back in 2009-2010. Unfortunately I no longer use the software program, Coin Manage, that I used then so I cannot access the file. But I did find it which is a miracle unto itself.
By the way, I did a pre and post slab weigh in to see if PCGS slabs are a consistent weight. Back then the answer was "No." So you cannot work backward from the slabbed weight.