Visiting Home and went through things with my parents as they move soon. The coin is most likely 1st-3rd century as we live along the German limes line.
Our garden is actually on top of an old Roman fort, they dug it up 50 years ago, catalogued it, and buried it again. The coin is most likely in the family longer than that though.
It looks similar to a sestertius is Antoninus Pius, but I can't find an exact match. There elephants face the wrong way and the legend on the reverse is different. Did you try /r/numismatics? They are better that I by a long way at this stuff.
Hi again,
Now I'm at a computer I can see the images a little better. This coin has the elephants going the right direction, but you see the "EX S C" on the reverse is in the wrong place, so this isn't a parallel, but it should point you in the right direction.
The obverse legend on both coins reads DIVA AUGUSTA FAUSTINA, identifying the bust. That's Faustina the Elder, the wife of Antoninus Pius. The "Diva" tells us she's dead when this was struck, 142 CE according to the acsearch.org. The troublesome "EX S C" means "by order of the Senate" which in the Republican period was used for special issues when the coins produced by the usual magistrates weren't sufficient for some reason. I don't know if it retains that meaning in the Imperial period. I don't know what the elephant drawn cart is supposed to symbolize.
The RIC reference for the example I found (again, NOT a parallel for yours) is RIC 1139.
