I think we are getting too hung up on looking at the backs of the cards. Elberfeld, Dahlen and Browne are all in the 150-350 Series or Print Group 1 because they were all in the first group. Dahlen Boston was just replaced with Dahlen Brooklyn on the sheets as the 150-350 Series played out.
The term "350 Only Series", or "Print Group 2" means something very specific. It refers to this group of poses:
https://t206resource.com/Print%20Gro...Checklist.html
That's all it means.
-Cobb red background was printed with a Piedmont 350 back, but it's a not a part of Print Group 2. It's in Print Group 5.
-Dahlen Brooklyn was printed with a Piedmont 350 back but it's not a part of Print Group 2. It's a Part of Print Group 1.
-Rhoades Arm Extended was printed with a Piedmont 350 back but it is not a part of Print Group 2. It's in Print Group 3.
The last bunch of posts are people looking at the same data and making up their own terms to describe what we're talking about. We don't need to make up new ways to describe this stuff. The Print Group framework is already established and available for anyone to refer to. Its simple and elegant and perfectly explains the timelines of the various series. Understanding the Print Groups is the key to a deep understanding of the patterns within the set.
Here are the checklists:
https://t206resource.com/Checklist%20Main.html