I think there are two possibility's in the differences in the Mathewson groups and it could be one or both.
One is a different print run and the other is a different printing facility (I'm pretty much convinced that the T206's were printed in different places that were either owned by or associated with American Lithograph.)
Besides the images another telltale sign is the captions.
I haven't done too much research outside the print group 1 subjects on this yet but the separation is pretty clear on them.
You can see the difference between the 150 and 350 printings on these Schlei examples. The 350 printings have a bolder/heavier and browner caption. The only exception is the Piedmont 350's which can be found with both versions. I think the thinner caption was the earlier and/or printed at a different facility version. You can also see this on the Mathewson's with the heavier/browner captions in the top row in Charles post.
Schlei catching back run - Copy (2).jpg
Schlei catching back run - Copy (3).jpg
Schlei catching back run - Copy (2) - Copy.jpg
Schlei Piedmont 350.jpg
Schlei Piedmont 350 backs.jpg