Why a Nobilitas reverse?

[mix_val]

Here I show off two rare denari of Severus Alexander.  Eastern Mint is clearly indicated by the style of lettering and portraits. I’d say the draped specimen is harder to find compared to the cuirassed, judging from the time it took me to acquire it! 

Obverse: IMPCMAVRSEVALEXANDAVG, Bust laureate right and draped (top coin) or draped and cuirassed (bottom coin)
Reverse:NOBI_LITAS (top coin) and NOBILITAS (bottom coin), Nobilitas draped, standing left holding onto scepter in right hand, palladium (?) in extended left. BMC 1055-6, C181. 
A previous thread suggested that instead of palladium, Nobilitas is holding Minerva/Athena. 
See:  https://www.forumancientcoins.com/board/index.php?topic=39237.0

What was the Nobilitas? According to “A Dictionary of Greek and Roman Antiquities” :
“This was the history of Nobilitas at Rome. The descendants of plebeians who had filled curule magistracies formed a class called Nobiles or men "known," who were so called by way of distinction from "Ignobiles" or people who were not known. The Nobiles had no legal privileges as such; but they were bound together by a common distinction derived from a legal title and by a common interest; and their common interest was to endeavour to confine the election to all the high magistracies to the members of their body, to the Nobilitas. Thus the descendants of those Plebeians who had won their way to distinction combined to exclude other Plebeians from the distinction which their own ancestors had transmitted to them.”
For more of this article see http://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA*/Nobiles.html

Here is my question for the experts.   The Nobilitas existed very early in Roman history but the earliest numismatic reference to nobilitas that I could find are those of the coins of Commodius.  Why?  Politics?  An appeal for support from the Nobilitas?  The Romans were very class aware.  Were the emperors up until then concerned with being members of the Patrician class exclusively?
Nobilitas has appeared on the reverse of the coins of Geta, Severus Alexander, Philip and Tetricus II amongst others.  The incorporation of nobiles into the obverse title (ie NOBC for nobiles caesar) was common for late roman bronze coinage of the Caesars but, as far as I could tell, it first appeared on a denarius of Herennius Etruscus. 

[284ad]

Nobilitas in this case is the personification of the idea of "nobility" itself rather than the Nobiles.

She carries a spear and an image of Minerva to indicate that nobility is achieved through both glory in battle and wisdom.

[mix_val]

Nobilitas in this case is the personification of the idea of "nobility" itself rather than the Nobiles.

Ok.  Instead of a political appeal, an attribute of the Emperor/Caesar.  But my question had more to do with its creation.  What social event causes a particular reverse to appear?  It's the idea that art is a mirror to the world.     
 

[curtislclay]

Why Nobilitas for Commodus in 187? Maybe because he was the first emperor "born to the purple", whose father was already emperor when Commodus was born in 161, and because he could trace his adoptive pedigree back to Nerva.

Why for Geta in 204? Hard to say. Geta was born a couple of years before Septimius became emperor, and had the same pedigree back to Nerva as Commodus, once Septimius proclaimed himself as a son of Marcus Aurelius in 195. Caracalla of course had the same claim to "nobility" as Geta, but by 204 he was already joint emperor. So maybe Geta's "nobility" was stressed in 204 as a sort of compensation for the fact that he wasn't yet Augustus like his brother and father.

Why for Sev. Alex. on Eastern denarii early in his reign? I'm inclined to regard this as mere rote copying of Geta's type by the Eastern mint, with little or no thought about its meaning and its appropriateness as a type for Alexander.

[mix_val]

Thanks Curtis.  I had the same thought about Severus Alexander's coins.  Makes for interesting collecting though!