Just the way Lockheed had separate Temporary Design Numbers and Basic Model Numbers... or like North American had Charge Numbers and Model Numbers... in the same way, Douglas used two different numbering systems. The first one started in the late 1920s and worked along the lines of NAA's charge number system: each number corresponded to a deal with a particular customer. Then there was the Model number system which was distinct. As with Lockheed, many observers and enthusiasts have mixed them up by using the word "Model" a little too lightly... Hence the apparent time lapses.