The Marvel-Schebler DD is a small updraft carburetor with a vertical air inlet. Production was taken over by Zenith after 1982. Most DDs are have aluminum throttle bodies and zinc float bowls. Some have an integral fuel pump.
DD carbs have a two digit carburetor number, eg. DD13, which is usually stamped on the carburetor body. Zenith-built DD carbs sometimes have a Zenith number stamped as well, or an aluminum tag.
Numerical List of Marvel-Schebler DD Carburetors