Hi Rich
All other subjects in the 350-only series are found with any one (or 2, or 3) or all 4 of these backs (CAROLINA BRIGHTS, EL PRINCIPE DE GALES, OLD MILL, POLAR BEAR).
Whereas, these 42 specific subjects were not printed with any of these regular 350 series backs. This is very unusual. And, what this implies to me is that this group of 43
subjects were printed on the same sheet, and quite early in the game (circa Spring 1910).
TED Z
T206 Reference
.