There was a photo in one of the earlier Parker books (Larry Baer author) showing a 20 gauge DHE built as one of two guns purchased by an east coast duck club in the Teens. It had factory 3" chambers...
Close but no cigar!! The pictures on page 36 and 37 of Larry's 1980 combined book of his two previous books from 1974 and 1976, are from the famous Widgeon Duck Club near Visalia, California. There were eight guns in that order to R.F. Cross Hardware Co., seven DHEs and one BHE. The gun Larry showed was the J.A. Hannah gun. Here is the hang-tag for the gun --
The tag shows it wasn't targeted with the hottest 20-gauge loads offered at that time of 2 1/2 drams of bulk smokeless powder or 20 grains of dense smokeless powder such as Infallible or Ballistite pushing 7/8 ounce of shot, but rather the lighter 2 1/4 dram load --
I don't know what Larry was talking about when he stated they were made "with 3-inch chambers for the then revolutionary "ajax heavy" 20-gauge magnum shells." The shells he pictures with the gun are Peters IDEAL with the standard load of the day of 2 1/2 drams of E.C. pushing 7/8 ounce of shot.