When was a pair of Greek and Roman gods first identified?

Question

There is a canonical correspondence between some Greek and Roman gods and goddesses, for example Ares and Mars. However, these two were originally different deities: Ares represented rage in war and Mars was also a god of agriculture, to mention a difference. My question concerns pairs of Greek and Roman gods like this, which were originally different but were later identified. This excludes Apollo(n), as this god seems to have been always the same, not identified later between the two mythologies.

What is the first written identification of a Greek and Roman god? How does the identification manifest itself? Is it explicit ("We call it Iuppiter, they call it Zeus"), is it a Greek story retold with Roman names, or something else? An unattested identification is also fine if it is referenced by a later source.

After months without an answer, I asked this question also on the mythology site.

Answer 1

I don't know if this is the oldest, but from Dionysius of Halicarnassus's Roman Antiquities 2.34 (first century BCE):

Μετὰ δὲ τὴν πομπήν τε καὶ θυσίαν νεὼν κατασκευάσας ὁ Ῥωμύλος ἐπὶ τῆς κορυφῆς τοῦ Καπιτωλίου Διός, ὃν ἐπικαλοῦσι Ῥωμαῖοι Φερέτριον, οὐ μέγαν (ἔτι γὰρ αὐτοῦ σώζεται τὸ ἀρχαῖον ἴχνος ἐλάττονας ἢ πέντε ποδῶν καὶ δέκα τὰς μείζους πλευρὰς ἔχον)

Then, after that triumph, and the sacrifice, Romulus built a new temple to Zeus (whom the Romans call "Feretrius") at the top of the Capitoline Hill. It wasn't very big—the old foundations still survive, and the longest side is less than fifteen feet long.

(Translation mine.)

This was the Temple of Jupiter Feretrius, the oldest temple in Rome, where the spolia opima were brought after a war. According to Nepos, it fell into ruin sometime before Caesar and was rebuilt by Augustus.