Strobilus, you seem to be sure?
I'm afraid so, Pierre. Just the glaze alone is a giveaway. I believe ancient
Firmalampen were very occasionally glazed but the glaze on your
lamp is very different to the vitreous lead glaze used during the
Roman period.
Also, from what I can see of the clay, your
lamp appears to be a modern high-fired ceramic (fired at a much higher kiln temperature than ancient
Roman pottery).
I suspect your
lamp is a museum reproduction, not a
fake intended to fool, but a usable souvenir that could be lit. The interior glaze is functional (to prevent oil leakage) while the exterior glaze has been artificially 'aged' to give it an olde worlde look.
BTW, you might find my website of interest:
http://www.romulus2.com/lamps/