Antoninus Pius, sestertius, Rome AD 153/4. Obv.: ANTONINVS AV-G PIVS P P TR P XVII, bust of Antoninus Pius, laureate, slight drapery on shoulder, left. rev.: LIBERT-AS COS IIII / S C, Libertas, draped, standing right, holding pileus in extended right hand and extending her left hand RIC III Antoninus Pius 916D There are not many examples of this left facing bust variant. The OCRE online database illustrates only two, and I could find none on ACSearch, only one on Vcoins. For the same coin, with the same legends and same reverse, the right facing head is very common... The left facing bust (with its slight drapery on shoulder) seems to have been a special limited series. But can anybody answer this question: why is this bust facing left? Is there a special signification? Thanks...
We know from Beckmann's die-linkage studies of the aurei and sestertii of Faustina I that left-facing varieties appear paired with many different reverse types. We also know that they were produced simultaneously with the more common right-facing bust types. They don't seem to have any particular significance and I consider them to have been produced on the whim of the die-engraver. Yours seems to be an obverse die-match to the British Museum specimen.
... and also to this one seen on Vcoins https://www.vcoins.com/fr/stores/ro...bertas_ric_916d_rare_type/955322/Default.aspx I wonder how many left facing dies are known for this sestertius...
The literature suggests at least two. A plain laureate head is described by Cohen (no. 537; RIC 916c) and reconfirmed by Strack, citing specimens in Munich, Paris, and University of Bologna). I haven't been able to find any examples online of this type, however and the museum collections cited by Strack haven't been digitized and Strack doesn't illustrate them, either. Cohen also notes a specimen in the BnF with a "lauré et drapé" (laureate and draped) bust (no. 538, RIC 916d) but this isn't confirmed by Strack, who generally cites the entire holdings of the BnF. I have not been able to find an example of a fully draped, left-facing bust type of this coin, and I think Cohen is probably referring to the type with the drapery on the right shoulder illustrated here by your specimen, the one in the British Museum, and the one at Romae Aeternae's V-coins store. There is another with this same obverse die in the Münzsammlung des Seminars für Alte Geschichte der Albert-Ludwigs-Universität in Freiburg.
Maybe the left facing coins were struck on “unlucky” days or for some other reason. But that’s sheer speculation.