According to Livy (8.17.3-4) these were appointed under the Consuls of 334, but abdicated when declared faultily appointed. In Chr. 354 is found the note: hoc anno dictatores, non fuerunt (consules). This is the first of the four "Dictator" years, 333, 324, 309, and 301, which were listed in Fast. Cap., but appear in none of the rest of our sources, though the Dictator and the Master of Horse in each case are listed in Livy under the Consuls of the year preceding (334, 325, 310, 302). See Fast. Hyd.; Degrassi 107, 410f., and esp. 110 on 309 B.C. (Broughton MRR I)
On these and the magistrates of the other Dictator years, 324, 309, and 301, see A. Drummond, The Dictator Years, Historia 27, 1978, 550-572: they were an invention which appears first in connection with the Liber Annalis of Atticus (Cic. Brut. 72; cf. 13-15), and may perhaps have been used to justify Caesar's annual dictatorships. See MRR 1.141.
(Broughton MRR III)