Cicero, In Catilinam, 3.19:
You remember, of course, that in the consulship of Cotta and Torquatus a large number of objects on the Capitol was struck by lightning, images of the gods were overthrown and statues of men of old overturned and the bronze tablets of our laws melted; even the statue of Romulus, the founder of Rome, was struck--you remember that it was a gilt statue on the Capitol of a baby being given suck from the udders of a wolf.The consulships of L. Aurellius Cotta and L. Manlius Torquatus were in 65 BC, and provide us for a firm date when the Lupercalia was struck by lightning.
Cicero, De Divinatione 1.20:
Here was the Martial beast, the nurse of Roman dominion, suckling with life-giving dew, that issued from udders distended, children divinely begotten, who sprang from the loins of the War God; Struck by lightning she toppled to earth, bearing with her the children; Torn from her station, she left the remnants of her feet as she fell.Cicero, De Divinatione 2.47:
'The statue of the infant Romulus,' you observe, ' was struck by a thunderbolt; hence danger was thereby predicted to the city which he founded.'I find it interesting that all the sources for this group are from the same writer, and that it was important enough to survive antiquity, but was not mentioned by more writers - but then again, there are very few sources for the Parthenon.
If the wolf with the head turned towards the twins was the Lupercalia statue in the Forum, then Cicero's Capitoline group presumably was the one where the wolf's head was turned away from the twins.