It is not proved that this bit of stone came from the Parthenon.
Debated. A small carved marble drop matches old measurements of the Parthenon's upper stonework.
The piece comes from the wreck of the ship Mentor, off the island of Kythera, Greece. The ship sank in 1802 in about 23 metres of water, near Avlemonas. The fragment is 9.3 centimetres long and 4.7 centimetres wide. The drop is 6.51 centimetres across and 2.2 centimetres high. The page says this is the first bit of carved building decoration found in the ship's cargo since the digs began.
More study is still needed before the exact building can be named.
https://www.ancient-origins.net/news-history-archaeology/elgins-mentor-shipwreck-00102607