Scott - The closest explanation we give to what you're looking for is on the
Print Group 1 page on T206Rsource.com.
It states the following about the Sovereign 150
Original 150 Subjects
Throughout the T206 set, the Sovereign back brand is an important key for understanding the set's composition. During the early production of the 150 Series, subjects from group 1 were printed with the Sovereign 150 back. Though this group totals 159 subjects, the Sovereign 150 checklist is complete at 150 subjects. This Sovereign checklist
is thought to be the original 150 subjects in the set, with the additional nine being updated or added to the print group after the Sovereign 150 printing -- or in the case of Magie/Magee, corrected prior to this printing.
We would love to breakdown every single conclusion that we draw on the site to it's most minute detail, but we felt it best to keep as much as possible simple and to the point.