HI, A little history lesson this morning, and I love history. Heisey started using their trademark an H in a diamond in 1900 and continued to use it until they closed in 1958. Imperial bought the existing molds then and made some items, mostly items Heisey was making when they closed. In 1968 Imperial announced they would no longer use the mark. In no way did Imperial change the Heisey mark.
Now to the tumblers. When Heisey first started using their mark, it was large. The story goes that salesman complained and the mark was made smaller. Hence, large mark, early piece. In later years I have always felt that Heisey made a game of their mark. On a lot of stemware it is so small and in such obscure places you can spend several minutes trying to find the darn thing.
So my conclusion is this, if they have a large Diamond H, they are definately Heisey and early. A pic would be wonderful, Terry