I was overlooking the size of your coin, which completely changes the diagnosis!
The primary determiner of
denomination is the size of the dies, not the
weight or size of the
flan.
The dies of your coin are c. 30 mm broad: therefore they are
sestertius dies. Your coin is exactly
RIC 592, but struck on an unusually light
flan.
If the metal of your coin is brass (yellow), then it is a
sestertius struck on an underweight
sestertius flan, the most likely solution; or possibly a
sestertius struck on a
dupondius flan. If the metal is copper (
red), then it is a
sestertius mistakenly struck on an As
flan.
The coin I ceded to the BM is quite different: it is a proper As, struck from dies c. 25 mm in
diameter. According to the BM website its inventory number is 1992.0509.266, and its
weight is 10.11g.
Unfortunately I do not have access to an image of the coin. It is not illustrated on the BM website, and apparently I did not make a plaster
cast of it before
selling it to the BM.