To D or not to D, that is the question
If I remember, a D-belly was more likely to be a 5- or 6-weight. That letter system designated diameters. Trouble was, Ashaway's D, using different plastic coatings, might or might not be the same weight as Cortland's D belly. The cry went out for a system based on line weight, and so the AFTMA system that we use today came to be, somewhere around 1960.
A designation like HDH was a double taper; one like GBF was a weight-forward. I seem to remember that there were designations like GAAF or G2AF, which was a saltwater-size WF line.