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.