Antoninianus - Probus Cuirassed, left, spear and shield; MARTI PACIF; Mars

RIC 543 was minted by the 4^th officina (Δ/-//-; -/-//ΔXXI) of the Ticinum mint. The original RIC text mentions 1 variation/bust type (bust types according to Pierre Bastiens grouping of bust types; see illustrated: probvs.net) and 2 variants are known: 

1. Radiate, cuirassed, left, with spear and shield (F1 bust (example: probuscoins.fr #4034), Numista #292896);
2. [var] Helmeted, radiate, cuirassed, left, holding spear in right hand and shield in left hand (E1 bust (example: probuscoins.fr #1211), not in Numista); 
3. [var] Radiate, heroically nude, left, holding spear in right hand and square shield (aegis) in left hand, from behind (F19 bust (example: probuscoins.fr #4523), not in Numista).

According to Pink, RIC 543 – with its obverse lettering nr. 4 (‘VIRTVS PROBI AVG’) combined with “MARTI PACIF” and exergue-/field mark Δ/-//- or -/-//ΔXXI – is part of the 6^th emission, series A (without value mark ‘XXI’) and B (with value mark ‘XXI’), both series being minted in the year 279. See: Karl Pink, Der Aufbau der römischen Münzprägung in der Kaiserzeit VI/I, Wien, 1949, p. 52-53. Pink points out that the Ticinum mint only during the 6th emission temporarily switched to using Greek symbols for its 6 officinas (A, B, Γ, Δ, Є, ς).