Although very common as a series, this Aradian bronze dated year 21 is only the second coin to my knowledge with this particular date. The other can be found in the BMC.
An 21 (239/8), d dans le champ en haut
1417 *D18-R37, Londres, BM, BMC 91. 3,46 g, 16,6 mm, 6 h. (left image)
1417b *D18-R37b, Stockholm, collection Rowe. 4.34 g, 17,73 mm, 6 h. (right image)
Phoenicia, Arados 239-238 B.C
AE 17.73mm (Thickness 2.35mm), weight 4.34g, die axis = 6h (180 degrees), denomination C.
Obverse: Turreted head of Tyche right, border of dots (Beveled flan).
Reverse: Prow of galley left with (Ἀθηνᾶ Πρόμαχος) Athena figurehead fighting left with shield and spear, AP monogram above, border of dots (of the flan). Aradian era date 21.