This coin is part of the famous coded Eqviti series (see the letter "E" in left field, which is the 1st letter of the EQVITI word struck by the first officina, i.e. officina "P"), 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:IMP C PROBVS AVG
REVERSE: CONCORD MILIT
BUST TYPE: H2= radiate bust left in consular robe, holding eagle-tipped sceptre (scipio)
FIELD / EXERGUE MARKS: E/-//PXXI
WEIGHT 3.95g / AXIS: 12h / WIDTH 21-23mm
