In the Three Emblems Law Volume 1, Section 3 mentions about Trat town as follows.
The emblem of Chaophaya Thammaraj Dechachat Ammatayanuchit Piphitworaphong Pakthathib Sawarathibodee Sriratana Kosathibodee Aphaipiriyabora Krompahu was a crystal lotus emblem. When there was a royal order appointing the suitable official to govern the town that dependent of Kosathibodee (Lord of Treasury), which are Chantaburi, Trat, Rayong, Bang Lamung, Nonthaburi Samutprakarn Samut Songkram Sakornburi. There were emblems that appointed governor, vice governor at the town dependent to Lord of Treasury, brought emblems to official duties and make sure good living to the town dependent to each Lord of Treasury, including tax setting for all the customs.
This paragraph shows that Trat town was a significant town in the Kingdom of Ayutthaya
Museum in Trat