You're correct.Buddha images of the Ayutthaya style can be divided into 4 sub-period. At first the second and third U-Thong styles of Buddha image were popular in Ayutthaya. In fact the large Buddha image at Wat Phanan Cheng,Luang Phor To,which was built 26 years before the foundation of the city, belongs to the second period of the U- Thong style.The real Ayutthaya style began about the middle of the 15th century in the reign of King Boromthailokanart.