Jerry,
I've got a No.8 Griswold skillet with the rib. Can't recall if I have seen the 6 or any other sizes. Yes, they would be the earliest smooth bottom block TM skillets. Have you noticed that the TM is just slightly different in proportion to the later block skillets. That has led me to think that Griswold may have offered both the smooth bottom skillets and the heat ring skillets at the same time. Catalogs do not show this and it is only a possibility.
My thinking is that the slant/EPU smooth bottom skillets sizes 2, 5, and 8 predate the smooth block TM pans but the blue book disagrees. A 3 and a 9 are also listed in the blue book as having been made in smooth bottom slant. The No.3 I doubt was ever made and I don't recall seeing a No.9 for sure. What they call the No.3 has a hint of a heat ring and I think that it is a heat ring but very low and different than the usual Griswold heat ring.
Is the rib on the 6 and 8 with a flat area rather than the sharp rib of the earlier block heat ring skillets? Some block HR skillet also have the flat rib. Handles are very interesting to me on Griswold skillets. Something should be done up on them. If I ever get around to getting a digital camera I hope I can provide some photos of handles.
Can you come up with some photos of the pans you are asking about or a link to an ebay auction?
Steve