The form or shape matches a genuine
Roman one -
type IIIa in the Aquincum (
Budapest) ring
catalogue typology.
One caveat. The idea that a
roman numeral on a ring equals a legion number is entirely unproven. It is only guesswork and speculation among dealers and collectors. As far as I know, there is no other proof - no historical reference and not even any indication that such
rings are most commonly found in soldier's graves or on
military sites.
By contrast we do know that when
Roman legions are referred to in a proven and unambiguous way, for example on roof tiles or belt sets or
engraved on other gear, they are always LEGIII or LIII, not III.
In other words, a III could stand for anything - the month someone was born in, the number of children they have, their apartment number, etc.
We all want to believe it means legion number because that is the coolest answer.
SC