In theory it is from the fourth series. However, the
reduced size and off-set strike mean it might be
imitative and from the time of the fifth or sixth series.
Bastien's series must be re-dated given the re-dating of Nepotian's usurpation.
His third series should be re-dated to September through December 351. But taking into account the fifth and sixth series an ambiguous coin like yours should be dated September 351 to the end of 352.
Details of the re-dating can be found in my
Koinon article, which is now available on academia.edu
https://www.academia.edu/41214464/Redating_Nepotians_Usurpation_and_the_Coinage_of_Magnentius
SC