That was my first thought too. High
medieval. For example, the Museum of
London book "Dress Accessories 1150 - 1450" includes this
type as a "double-oval frame buckle".
However, the tongue looks very, very
Roman. I can't find a tongue with that kind of moulded and incised design on any
medieval example!
Also, many late Roman-era buckles, especially Germanic, consisted of a long and thin single oval with a similar tongue.
So, assuming the tongue is original, I would personally call it: undecided - possibly high
medieval and possibly late-Roman local/provincial
style.
Hopefully we can find either a
Roman example of the double-oval or a
medieval example of that tongue
style.
SC
PS - If it looks possible that the tongue was added after excavation then maybe you have
medieval buckle and
Roman tongue.