...the blue book has slant epu at 1910 to 1929, Next it has block epu at 1920 to 1930. Note, there are 9 years that over lap assuming these years are right. Actually 10 if the slant ended at the end of '29 and the block started at the start of '20. I suspect that in those overlapping years the slant was produced more because the block was the 'new kid' on the block. (pun?) If the dates are correct and my guess is anywhere near correct, that leaves only 1 year that the block epu was made exclusively, except that the block epu smooth bottom also started in 1930 to 1939.
Carol and C.B.
I don't fault your reasoning C.B. on the dating but I don't agree at all with the dates give for many pieces or the range of dates in the blue book. If you haven't already, look at "Griswold Trademarks" under Educational Information to the left of this page.
My interpretation of the slant TM taken from the US Trademark site is that it was first used in commerce in 1909. The block TM seems to have first been used in 1925 though I wonder if it wass more like c.1922.
The Hearts Star waffle iron was patented (design patent) in 1920 and uses the slant TM while the 1922 patented waffle irons have the block TM.
In any case, I would say the slant TM was first used in 1909 and the block anywhere from about 1921 to 1925. As for any overlap in selling the slant TM and Block TM pieces I think there would be some overlap but not that much, especially on popular pieces like skillets (at least in the more popular sizes).
I see the end of the slant/EPU skillets being about 1922-24 at which time the block TM heat ring skillets came in. They probably lasted until sometime around 1930 but I don't know anyone who really knows or how anyone would find out. Block TM smooth bottom skillets may have gone from c.1930 to about 1940. Small TM came out in 1939 I think but on the hammered pieces and hinged pieces. Again, very hard to pin down dates since most catalogs don't show the bottom markings or trademarks in their illustrations or photographs.
If the slant TM was used well past the 1922-1925 period I would think it would have been on other than skillets and for less popular pieces. There are some slant TM skillet griddles that are porcelainized that date from the 1950's I think so the slant TM did not go completely away for many years on a few isolated items.
Back to plain skillets; I would wonder why there would be any overlap to slant and block TM's other than maybe a year or why any overlap more than a year between the block HR and block SB. I only wish the book(s) had given some documentation to their dating. I have talked to Dave about dating in his books and he has told me that the new blue book will have some dating revisions. As to the dates that I, personally, believe to be correct I will have to say that they come from my own observations for the most part over 30 years. I try to come up with what would be logical but, in doing so, perhaps I may have overlooked some information or just be plain wrong at times. I'm always open to new information that is based on some substance and, better yet, there is some proof.
Steve