I have two charts (one from Mike Orlen and another from the late barrel guru Stan Baker) that match quite closely with Little Creek's numbers for the 20ga.
However, there was significant variation from one maker to another. Belgian Brownings, for example, tended to be choked tighter than guns from other makers. In his "Shotgun Book", Jack O'Connor listed choke constrictions used by Browning, Winchester, and Remington. Browning's 20ga full choke constriction was .033, while Winchester's was .027. Browning's 20ga full choke actually had more choke than Winchester's 12ga full (.031). And anyone who has measured constrictions on a Parker Reproduction 28ga will find that the Japanese must have been using full choke numbers on those guns. WAY tighter than standard.