Rod's silver Roman - note the huge stress crack on Reverse
I'm reasonably certain this is RSC/Babelon "Lucretia 1" - Cn(aeus) Lucretius Trio, 136 BC. Everything visible on your coin fits with the descriptions, line-drawings and photos I have available. There are a couple other possibilities - not quite such complete matches, but possible - from approximately the same time-frame. Unfortunately, the remaining "scales" on this coin cover the two areas I'd need to see to be 100% certain and rule out the possibles. There should be, in addition to the visible "X" below the chin: "TRIO", vertically & downward, behind Roma's helmet on the obverse. On the reverse, the whole abbreviation beneath the horses of the Dioscuri should be: "CN LVCR".
This type is also # 114 in vol I of the recent edition of Sear, Syd/CRR 450, and RRC 237/1a.