Wow, I am very very new at Rhino and 3D modeling. I was tempted to respond with “can you hold my hand through these steps!”. Ultimately it was easy, but took me lots of trial and error time with each command.
This problem taught me how to ExtractSrf, Untrim, Trim, Join, and Cap. I exported to STL and Simplify3D slices the model correctly now.
With this solved, I decided to Extract the number, and recap the top. I am printing 12 of these dividers, with the tab shifting for each one. Looks like I need to create each model, with each requiring the steps you outlined above to fix this inner corner(s) issue. Guess I could have left the number. I just realized there isn’t a good way to just move the tab and save each iteration.
Jim, thank you very much for taking the time to steer me in the right direction!