I find this coin pretty interesting. The style looks good, so I believe it could be official. The first problem is the reverse legend- CAESARVM NOSTRORVM for a Constantine I obverse. So it would seem to be a mule, but the mintmark (TAR) is not listed either. T star AR is recorded, so maybe the engraver left out the star, but in the footnotes of p261, RIC VII mentions several coins like this- "coins of otherwise regular appearance of irregular m.m." I have also seen several other examples with a similar mintmark, so maybe this mintmark deserved to be listed in RIC rather than just being included in a footnote, of course, it would be helpful to examine and compare many more of this type.
Constantine I
A.D. 322- 323
18mm 2.9gm
CONSTAN-TINVS AVG, laureate bust
CAESARVM NOSTRORVM in laurel wreath VOT X.
In exergue TAR
cf RIC VII Arles 252/254