Style and
fabric look official, but one has to suspect an ancient imitation, since as you say there is a seven-year gap between the two sides of the coin, the
obv. being of TR P XIX-XXI, but the
rev. of TR P XI-XII.
Authentic
mint mules generally couple dies of different personages that were in use simultaneously, for example an
obv. of Pius with a contemporaneous
rev. of
Marcus Caesar, or dies of the same emperor that belonged to directly successive issues, for example BM 520 pl. 11.14 that you mention, with
obv. IMP II but
rev. COS IIII Annona standing.
When
IMP II was added to Antoninus'
obv. legend, TR P XIX should have been transferred to the
rev., producing the
rev. legend TR POT XIX
COS IIII. The BM coin was struck from a
rev. die that
had been
engraved before the rearrangement of the titles, and that
had then mistakenly not been retired from use when that change was made.
For your coin to be official, a
rev. die from TR P
XII would have to have been retired for seven to nine years, then brought back into use when Antoninus was TR P XIX-XXI. That seems unlikely to have occurred!