Jochen's explanation is correct.
Strack saw the coin in the stock of the
Paris coin dealer
Feuardent, so
Mattingly had certainly not seen a specimen!
One thing I have to look into: is the
legend SECVRITAS alone on coins, not followed by AVG or
IMPERI or P R, so unusual that
Mattingly was justified to propose a correction to
SECVRITAS AVG?
By chance, I acquired a specimen of this
sestertius myself about a year ago. Unfortunately it is worn; on
rev. [CO]S II can be made out faintly in
exergue, but [SECVRITAS] is illegible.
My specimen is from different dies than the one shown by Jochen, and its
rev. type varies in two small details. My
coin shows a strut between the legs of Securitas' throne, omitted on Jochen's specimen. On Jochen's coin
Securitas seems to be supporting her
head with her l. hand; on mine her l. hand is much farther from her
head, as though she were perhaps drawing up her drapery. However, the gesture of supporting
head in hand is typical of
Securitas, so that is probably what was meant on my coin too.