Some years ago,
on a text driven newsgroup with litte access to images,
we were confronted with this query,
What was this weird pmk on a German stamp?
It took a group of us weeks then, to track down the culprit,
(or assumed route it would have taken to get this pmk)
A letter had been sent from Germany to Scotland
and the stamp possibly had not been cancelled in Germany
so the clerk in the Scot village decided to cancel it,
He used a a machine invented by Rowland Hill's brother
called a "parallel motion cancelling machine"
with an impression of a Scot "Brunswick Star"
I'd suggest the same thing may have happened to yours
of Siam, just an over ambitious postal clerk?