These images of the stamps have been adjusted so that the postmarks may be seen more clearly.
The first issue of Siam was not supplied to many provincial post offices outside Bangkok as most of them opened after the second issue became available in 1887. These early provincial postmarks on the 1 Solot blue are, therefore, suspect..
Those from Klong Rangsit and Chieng Kham are well-known. They were applied fraudulently by an employee of the Post Office, probably after 1900, when these stamps were declared void for postage and sold off to collectors as remainders.
The fraud came to light when a German ex-employee of the Post Office was interned in 1917, as Thailand joined the First World War on the other side. His papers showed that he and a predecessor had been cancelling stamps supplied by a European stamp dealer. Postmarks from Chieng Kham and Klong Rangsit are specifically mentioned in connection with this fraud.
Yasothon post office, I believe, did not open until after 1900, although I have no firm evidence of this. If this is so, genuine postmarks from Yasothon on the first issue would be impossible.
Chieng Kham post office opened on 15 July 1905 and closed in 1909, so genuine postmarks on the first issue are not possible.
Klong Rangsit post office opened on 20 June 1897, so genuine early postmarks on the first issue are possible, although they are most unlikely.