Crawford's catalogue in RRC includes 6 major types, 4 fully anonymous(i.e. no mintmark or name), along with L and ROMA monogram mintmarks. A 7th, with mintmark "H" has been published since then. All except for the common "44/7" are very rare to extremely rare but I've got examples of all of these types in my photofile along with all known styles of the common 44/7 and this sestertius matches none of them. Even so, it looked genuinely ancient to me, for whatever that's worth, so l went ahead and purchased it. While it was in transit I reached out to several RR collectors who have been studying these coins much longer than myself and received a variety of responses. All at least agreed it was ancient, but there were several ideas on what it could be: An imitation struck in good silver but on a very thin flan An imitation struck on a flan of debased silver A plated (fourree) imitation An unpublished official type from a branch mint The only real point of agreement was the likely source: Spain. A lot of strangely styled Roman Republic coins from Second Punic War era mints, as well as some coins that are almost certainly imitations, are believed to have been minted in Spain. I think the collectors were probably spot on with the Spain hypothesis but unfortunately the seller refused to provide any more information about the origin of the coin so I had to wait until it was in-hand to investigate further. Once the coin arrived I immediately noticed not only how light it was but how thin it was. I've attached a picture of it side by side with an official 44/7 sestertius. This coin is on the right. It is not only much shallower in relief but much thinner in depth. I also examined the dark, chipped spot on the edge. Under low magnification it does not appear to be bronze, but instead I think it is simply some remaining "find patina" which, due to its positioning within the crack, has not been cleaned off. As shown above, the coin is noticeably thinner than a normal, definitely official sestertius. This at least allows the possibility that the coin is not plated or debased and that the weight is simply due to the much smaller flan. The granularity of the surfaces, especially near the edge chip where you can see individual grains of silver is something I've only ever really seen on high purity silver coins, so I can't help but think it might still be of relatively good silver. Unfortunately I have no way to measure the specific gravity of a coin as small as this one, so additional information on the material makeup of the coin will probably have to wait until I find either a lab that can reliably determine it's specific gravity or perhaps even schedule a neutron activation analysis at a local lab which would reveal the silver purity to a very high confidence level. My personal opinion, for whatever it's worth, is that it is some sort of imitation, probably produced in Spain in solid, but perhaps debased, silver. I think the chance that any official Roman mint made this coin is very low, but I do think there is a small possibility given how strange some of the denarius styles were that came out of Spain during the war with the Carthaginians. As always, feel free to post anything relevant. And as mentioned above, I'd love to hear what you think this coin is.