Antoninianus - Probus Helmeted, cuirassed, spear and shield, from behind, left; CONCORD MILIT; Concordia

Reverse

Probus, standing right, clasping hand of Concordia, standing left.
In exergue XXI followed by officina mark or in exergue XXI and officina mark in field.

Script: Latin

Lettering: CONCORD MILIT

Unabridged legend: Concordia Militum.

Translation: Harmony with the soldiers.

Comment

RIC 650 was minted by the Siscia mint by all officinas except for the 7th. The original RIC text lists multiple variations/different bust types, listed below (bust types according to Pierre Bastiens grouping of bust types; see illustrated: probvs.net):

1. radiate, draped, right (bust A, (*), Alföldi 026.118 (**), probuscoins.fr #3717, Numista #293655);
2. radiate, cuirassed, right (bust B, Alföldi 026.124-127, probuscoins.fr #3247, Numista #293653);
3. radiate, helmeted, radiate, cuirassed, left, holding spear in right hand and shield in left hand (bust E1, Alföldi 026.143-157, probuscoins.fr #3468, Numista #407095);
4. radiate, helmeted, radiate, cuirassed, left, seen from behind, holding spear in right hand and shield in left hand (bust E2, Alföldi 026.166-167, probuscoins.fr -, this coin);
5. radiate, wearing imperial mantle, left, holding scepter surmounted by eagle in right hand (bust H2, Alföldi 026.103-117, probuscoins.fr #420, Numista #293006);
6. radiate, cuirassed, left, holding spear only (bust F5, Alföldi 026.132-314, probuscoins.fr #4702, Numista #407094);
7. radiate, cuirassed, left, holding spear in right hand and shield in left hand (bust F1, Alföldi 026.135-146, probuscoins.fr #3673, Numista #293657);
8. radiate, cuirassed, left, seen from behind, holding spear in right hand and shield in left hand (bust F8 (***), Alföldi 026.158-165, probuscoins.fr #3460, Numista #407093);
9. [var] radiate, draped and cuirassed, right (bust A2 (*), Alföldi 026.119-123, probuscoins.fr #3551, Not in Numista);

(*) RIC seems to only list the A bust (Probus, draped, right) and not the A2 bust (Probus, draped and cuirassed, right). See for example RIC 651 where the A2 and A bust are separately listed. Alföldi lists both the A bust and the A2 bust in (**) Andreas Alföldi, Vorarbeiten zu einem Corpus der in Siscia geprägten römischen Kaisermünzen. Heft V. Verzeichnis der Antoniniane des Kaisers Probus, Budapest, 1939). (***) The F8 bust can be cuirassed – as described in RIC – but also draped as for example: probuscoins.fr #3719.

As is clear from Alföldi's list (see 026.103-167), probuscoins.fr and probvs.net, there are multiple variants of RIC 650, in bust type (for example the A2 and B10var busts) as well in mint/exergue mark (XXI but also ++I). 

According to Pink, the coins of RIC 650, were minted as part of the 2nd (year 277, mint-/exergue mark style: Γ//XXI or ς//KA), 5th (278, -//XXIT) and 7th (280, T//XXI) emission (Karl Pink, Der Aufbau der römischen Münzprägung in der Kaiserzeit VI/I, Wien, 1949, p. 39).

RIC also lists somewhat remarkable mint/exergue marks: 

• Ч//XXI. In the 1st emission, the letters VI – for the 6th officina – where sometimes linked to form a Ч (a ‘retrograde’ μ). See for example: probuscoins.fr #1278 RIC 644 and #1281 RIC 716. Also see: probvs.net. Unsure whether Alföldi also means the Ч where he lists μ//XXI (for example: 026.105). 
• ꟼ//XXI is more of a misspelling, also listed by Alföldi (for example: 026.137), and an example is listed by proboscoins.fr (#4888) for the 1th officina in the 7th emission. 
• S//KA. This mint/exergue mark should be for the 2nd officina. There is no photo of an example listed anywhere, and it is not listed by Alföldi or Pink. Pink lists, as part of the 2nd emission for legend #3 (‘IMP C M AVR PROBVS P F AVG’), the mark ς//KA (for the 6th officina) being used.
• IV//XXI is also listed by Alföldi (for example: 026.107 and 026.143). This is exceptional for the 4th officina. See: probvs.net. It is not listed by Alföldi or Pink.

Interesting fact