Since you are building a 9, one presumes you are talking O-320? If so, the D1A is an A1A with Constant Speed prop provision, 25 degree spark advance, Bendix mags, high compression, 7/16 prop attach bolts, straight riser in oil sump, and -32 carb.
A D2A is a D1A with fixed pitch prop and 3/8 prop attach bolts.
This info from a certified Lycoming engine list, which if I recall correctly I got from Dan C.'s website.