Curious: Weight (g), and diameter (mm), also, would you be able to take a pic of it on edge? (Checking for any casting seam...) So far it looks pretty good... let's check the specs... Andrew McCabe shows a pic of this as Crawford 387/1 (Sear 318) struck in 77 BCE as L Rutilius Flaccus as moneyer... I am not an expert by any means, but this is how I start chasing down info.
This one is in my collection: Roman Republic L. Rutilius Flaccus. 77 B.C. AR denarius (17.8 mm, 4.09 g, 7 h). Rome mint. OBV: FLAC, helmeted head of Roma right REV: L · RVTILI in exergue, Victory, holding wreath and reigns, driving galloping biga right. REF: Crawford 387/1; Sydenham 780; RSC Rutillia 1
It is a little heavy for this time, but in tolerance. I think everything looks ok. The bankers mark looks legit. Any seam when looking on edge? Otherwise, personally, it looks legit to me from the photo.
Looks good to me. Weight is within expected range, style is good, I see no signs of toning and the big hook banker's mark is extremely sharp and would not be translated so well if the coin was a cast, even a relatively high quality cast can't generally transfer a banker's mark so sharply. So I think it's absolutely authentic.