Brian,
Here is a rough try at categorizing the Wagner DO's
First ones were like the early early pots with a flat top. Very nicely cast; probably better than the ERIE's
Next(?) came a pot with a humpbacked domed cover that had an iron knob that was nickel plated and that you could put your finger through from the side. No basting rings. Bottom of pot marked Wagner Wager
Next was a similiar shaped cover with integral loop iron handle and this cover had basting zig-zags (and are they hard to clean!) Cover top had incised writing on it. Bottom of pot was either Wagner Ware or Wagnerware with the large "W". Marked ROUND ROASTER. No catalog number. (note that wagner referred to the number on their pots as catalog numbers while Griswold called theirs pattern numbers. Pretty much the same purpose I believe).
Next version had the domed cover with the 1" high sides and zig-zag basting rings and lots of patent dates inside the cover and large raised writing on top of cover. Pot marked Wagnerware with the large W but early versions might have been Wagner Ware with separate W's. I don't think there was a catalog number yet. Also marked ROUND ROASTER. Some with this cover may not have been marked with ROUND ROASTER, I am not sure.
C.1930's came the cover with the shallow zig-zag basting rings (easier to clean). I am not sure what, if anything, was written on the cover's top or if it had those concentric circles. Pot not marked ROUND ROASTER and I think the catalog number was there. Shallower incised writing. No raised writing on top of cover.
Late 30's (?) is the pot with catalog number on bottom. Cover has shallow circles incised on top of cover and the basting rings are circular with notches in them. No writing on top of cover.
There is a later version or two but with no collector value and my mind is hazy on the facts for these.
That should give you a start and you can look at photos and make corrections and additions. Please let me know and I'll update this post. I am going from memory and, though I have probably had all the Wagner do's over the years, I no longer do and have not been looking for them in particular.
The earlier No.6 Wagners are neat pieces. I like the early style with the knob on the cover though the casting has a somewhat rough texture at times.
Steve