This coin is part of the famous coded Eqviti series (see the first letter "I" in left field, which is the 4th letter of the EQVITI word struck by the 4th officina, i.e. officina "Q"), discovered and first published by Alexander Missong in his articale ‘Gleichartig systemisirte Münzreihen unter Kaiser Probus’ (1873).
There were 6 officinae at the Ticinum mint at that time; each one struck a different reverse with a different letter from the EQVITI code.
For more information about the (A)EQVITI series see the below excellent article by Gert Boersema:
http://www.oudgeld.com/webbib/translation_codewords.htm
OBVERSE: VIRTVS PROBI AVG
REVERSE: MARTI PACIF = Mars walking left, holding olive-branch, spear and shield
BUST TYPE: E1 = radiate, cuirassed and helmeted bust left, holding spear and shield
FIELD / EXERGUE MARKS: I/*//QXXI
WEIGHT 3.76g / AXIS: 5h / WIDTH 21-22,5mm
