XXI

On Roman billion antoniniani of Probus and Aurelian, XXI is a mark of value indicating 20 parts bronze to 1 part silver (about 4.5% silver). On some coins the mark is written with the Greek numerals KA (K=20, A=1).

On Roman tetrarchy folles, XXI is a mark of value indicating 20 denarii communes = 1 follis.

On copper coins of the Vandals, XXI is a mark of value indicating 21 nummi.

DICTIONARY OF ROMAN COINS

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XXI - (1) with Latin letters - XXIP, XXIS, XXIT, XXIQ, XXIV, XXIVI, XXIVII, XXI, Prima, Secunda, Tertia, Quarta, Quinta, Sexta, Septima, or P, S, T, Q, V, VIXXI. Prima, Secunda, Tertia, Quarta, Quinta, Sexta XXI.
(2) with Greek letters XXIA B, G, E, S, Z, 1, 2, 3, 4, 5, 6, 7, or A, B, G, D, E, SXXI, 1, 2, 3, 4, 6 XXI - on coins of Aurelian and Probus [XX].

XXI, as a mark of value on the copper coins of the Vandals, without name of king.