RIC 863 (Vesp), BMC 305 (Vesp) var., RSC 106 var.
Rome Mint, 76 AD
Obv - T CAESAR IMP VESPASIAN; Head of Titus, laureate right.
Rev - IOVIS CVSTOS; Jupiter stg. facing, with patera over altar and sceptre.
Rated R2 by the RIC and unlisted previously, this was a most difficult coin for me to find.
Here is Curtis Clay's text from HJB's 159th buy or bid sale, which I cannot improve upon:
"A rare variant of the obverse legend for this reverse type, only recently published, allowing us to date the type's introduction precisely to 76 AD. In the course of that year, the obverse legend on Titus' gold and silver coins was expanded from T CAESAR IMP VESPASIAN to the same with VESPASIANVS, and the new form lasted until his accession as Augustus in 79. Titus' IOVIS CVSTOS reverse type was previously recorded only with obverse VESPASIANVS, so datable 76-79 AD; but RIC-863 and our coin with obverse VESPASIAN suggest that the type was introduced precisely in the year of the legend change, 76. RIC-863 cites two specimens of this denarius, in Berlin and in a private collection; we have seen a couple of others in trade or private collections over the past few years."
It's not everyday you come across a variant which can pin down the dating of a well known type! Not only is the coin rare, but the portrait on this example, in my opinion, is quite exceptional.