For me, coinage of Probus is quite a fascinating topic. The obverses of his coins display some of the most varied and detailed armor and bust type designs on ancient coins. Also, the mints had some fun with field marks and made series which corresponded with each other. For example, look at Ticinum, RIC 491 on the Ticinum sub-page. The star left and the I right are the emission signs (sometimes I left and star right): the 10th emission (282 AD). Very special for this emission are the letterings in the field: each officina has its own letter in the field and a different reverse type was attributed to each of the six officinae:
reverse type, letter in field, exergue mark
CONCORD MILIT, E, PXXI (prima/first officina)
PROVIDENT AVG, Q, SXXI (secunda/second officina)
SALVS AVG, V, TXXI (tertia/third officina)
MARTI PACIF, I, QXXI (quarta/fourth officina)
PAX AVGVSTI, T, VXXI (quinta/fifth officina)
SECVRIT PERP, I, VIXXI (sexta/sixth officina)
When you place the letters in the field according to the exergue mark in rising order (first P, then S, T, Q, V and VI) you get the word "EQVITI". Rome also produced this series, theirs being AEQVITI.
A note about coins from the Serdica and Tripolis mints - their exergues read KA(officina), which is the same as XXI(officina) everywhere else. Serdica had four officinae - A, B, Γ and Δ. Tripolis also employed the KA exergual mark, but the three officinae were marked as - star, crescent or wreath, and T.
Christophe Oliva has created a very interesting page showing various designs on the shields of Probus: http://www.fredericweber.com/articles/les_boucliers_de_probus.htm
To Dawson Lewis, Grzegorz Kryszczuk (GK), Steve Coulter, Bill Martin, Eddy Raes, Philippe Gysen and Martin Griffiths, I owe eternal gratitude for all of their assistance in understanding and attributing this complex series of coins. Thanks team!Martin Griffiths's Website: http://maridvnvm.ancients.info
Grzegorz Kryszczuk's Website: http://probvs.net/probvs/
Dawson Lewis's Website: http://www.mycoinpage.com