15th Century U-Thong Buddha Head - Type C
U-Thong A - Mon Type - Mon features and a simple small chingon like Ketumala similar to previous Lopburi & Suphanburi type images. Late 13th to mid 14th century.
U-Thong B - Khmer Type - Square jawed Khmer type features fused with a Sinhalese flaming type Ketumala. Mid to Late 14th century.
U-Thong C - Sukhothai Type - slightly more elongated Indic featured head mimicking the slender qualities seen in the Sinhalese influenced high period 14th century Sukhothai images from the north, again with a flame like Ketumala. Late 14th to mid 15th centuries. (The above image is Type C.)
The above datings are just a general guideline and not precise dates as images do crossover. Thai Buddhist art is complicated with much crossover & reinterpretation interms if styles and dates.