I'm no expert, but I assume you are referring to the catalogue numbers of early US Divers regulators. I found the following at http://aquaticcommons.org/15004/1/Histo ... 6_1998.pdf
HISTORICAL DIVER No.l6 Summer 1998
The first Aqua-Lung equipment catalog, printed in April of 1953, without a distributor's name, listed three complete unit models; Standard Model A consisted of a 70 Cu. ft. tank, riveted harness, "Aqualung" regulator, and a reserve valve; Navy type Model B used a twin tank block, regulator, and reserve valve; Junior Model C consisted of a 38 cu. ft. tank, regulator, and K valve. The Aqua-Lung regulator was listed as Model D and a hooka unit as Model S . Alphabetically the shut-off valve was listed under K, the reserve valve under J, and the positive orifice type valve under R. From that point on, these became known as K, J, and R valves. Early tank packs consisted of a harness only, later bands were added for durability.
The original Model D Aqua-lung regulator, which was later to become known as the DA, consisted of a lower box containing a large horseshoe lever and diaphragm, and an upper box for protection, opened to water pressure and containing the exhaust valve. Side clips held the two halves together, with the diaphragm sandwiched in-between. The hose assembly, which on early models used gum rubber hoses, was open at the mouthpiece and could easily be flooded. Later in 1954 non-return valves were added for easier clearing and use.
In late 1954, a full face mask from France called the Natascope was offered with the regulator. Designations were changed to read DB for the regulator with mask and DA for the regulator only. In 1955, catalog descriptions listed the regulator as the DA model. Later single stage models would carry the prefix DX, DY, and DW Mistral.
The above appears to explain the origin of the "D" in the "prefixes" DA, DW and DY. I'm sure others will fill in the blanks.