The "D" means that it incorporates changes that the "C" does not have.
According to note 5 all O-300 have dampers.
C145 series engines equipped with dampered crankshafts are identified by suffix letter "D" following the engine serial number which denotes one each 5th and 6th order dampers. O-300 series engines are also equipped with crankshaft incorporating one each 5th and 6th order dampers.
Note 6:
O-300-A is similar to C145-2 except parts material and ignition component substitutions. O-300-B is similar to O-300-A except incorporates crankcase and crankshaft provisions for use of a hydraulically controllable propeller from the engine oil pressure. O-300-C is identical to O-300-A except for propeller flange provision as indicated. O-300-D is similar to O-300-C except for provisions for Continental right angle automatic engagement starter drive which incorporates a vacuum pump drive. O-300-E is similar to O-300-D except for incorporation of governor drive pad and crankshaft provisions to supply governor oil to propeller
And Note 7:
C145-2, -2H, -2HP; O-300-A, -B: SAE-AS127 No. 3 flange O-300 C, -D, -E: ARP502 Type I flange 4-7/8 in. o.d. by .31 in. thick with six ½ bolt holes in 4 in. diameter circle. O-300-E has provisions for transfer of governor oil to propeller.