The style of the bust is sometimes the only way to differentiate coins from different emissions that otherwise share the same obverse legend, reverse legend and reverse type.
The earliest bust types resemble the coinage of Tacitus / Florian making use of previous dies / designs until details of the image of Probus could reach the mint. These early busts appear in the first and into the second emissions.
When referring to busts styles, there are two main reference schemes that are generally used. The two schemes are RIC (taken from Roman Imperial Coinage. volume 5, part. 2) and BMC (British Museum classification). In general this site uses the RIC types but this section includes both classifications for completeness.
RIC and BMC both include “Radiate, draped bust right” (RIC Bust type A, BMC Bust Type C1 or C2) and “Laureate, draped bust right” (RIC Bust type B, BMC Bust type C1*), it is likely that these bust types do not actually exist and were erroneously used to describe the radiate and laureate “draped and cuirassed bust right” (RIC Bust types C and D, BMC Bust types D1, D2 and D2*), where either the cuirass was missed or was unclear on the examined specimens and all references in RIC to Bust type A should be read as Bust type C etc.
The Antoniniani of the first four emissions are made up solely of the busts “Radiate, cuirassed bust right”, known in RIC as Bust type F, and “Radiate, draped and cuirassed bust right, seen from the rear”, known in RIC as Bust type C. These busts can be distinguished from similar, later busts in that the busts seem thicker in the neck and that there is less cuirass visible than in later emissions. This may seem like a vague description but it is easiest to explain with some examples.