Phra Buddha Sai Yat is the name of the 8 meter high by 37 meters long reclining Buddha which is usually draped in a large marigold robe but this was not the case when I visited. The vertical arm is a characteristic of reclining images made in the Middle Ayutthaya period - after the 16th century (Amatyakul 47). This is located at Wat Lokaya Sutha which is a massive temple ruin. There are two small chedi outside of its compound and a portion of the spire of a third chedi is still in situ.
The royal palace (former capital of Thailand) that once stood near the temple was levelled to the ground in 1767 during the Burmese invasion of Ayutthaya. Thailand