John, I just bought an arc Lodge #8. Mine has a notch. I have some other Lodge's with a raised number, solid heat ring, but no "lodge" logo on the back. (One of them has a butter mark there, as if it was their first attempt at writing "Lodge", messed up, and then covered up the unsightly mistake with a little iron.
My uneducated guess ... Lodge made skillets with no notch when they first started making skillets under the Lodge name, then started adding Lodge to the back, then started adding the notch, then took the name off of the back. ... Each transition had a period with characteristics of the last era and the new era.