No Homonoia from Philipappus listed for Mamaea! But there is the same type for
Severus Alexander & Maesa, HrJ (2011) 6.33.38.1. Because here the important parts
of the obv. legend are illegible, I think it is this type too, attributed in error to Julia
Maesa.
If that is the same coin as HrJ (2006), p. 194, third coin, then I think you are correct.
That coin on p. 194 seems to be from the same dies as your nice specimen, and therefore is of
Alexander and
Mamaea, not Alex. and
Maesa.
HrJ have taken their image from Blancon List 41, 2003, 491, where the coin is correctly described as being of Alex. and
Mamaea!
One wonders on what basis HrJ rejected this correct
attribution and misassigned the coin to Alex. and
Maesa, without any indication that the
attribution might be doubtful. Maybe just carelessness: they copied the picture from Blancon, without recording
his correct
attribution, then misattributed it themselves, unaware that they were contradicting Blancon!
The matter would have been easy to sort out, if HrJ
had only paid attention to the dies: under Philopappus at
Marcianopolis there was only one
obv. die for Alex. and
Mamaea, and one
obv. die for Alex. and
Maesa.
My photofile contains the Blancon coin, Jochen's example (dealer picture, Sept. 2010), and four others from the same pair of dies.