Luke would be a good person to chime in on this. One of the big factors is a lot of the portraits are print group 1 subjects and the majority of the low pop Old Mills are from print group 1.
Here are a couple of examples for comparison
Hooks Wiltse
Portrait no cap print group 1 PSA SGC pop = 9 Portrait with cap print group 4 PSA SGC pop = 55
McGraw
Portrait no cap print group 1 PSA SGC pop = 8 Portrait with cap print group 4 PSA SGC pop = 34
There is evidence that Old Mill was one of the last backs printed on the group 1 subjects and I think that is the reason many of them are low pop with Old Mill backs.
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