I frequently see in eBay
used Thailand stamps placed in the very wrong time period of 57 years earlier than when the postmark was actually applied.
Even today, Thailand does use the Buddhist calendar year in the cancellation of postage stamps.
In the Buddhist calendar the Christian era 1957 correspondents with the Buddhist year 2500. Seeing a Buddhist year, substract 543 to get the Christian year.
The error quite some sellers make on eBay
is that when the year in the postmark shows say 21, they assume its 1921. However its 2519 or 1976 !!!!
My guess but I am not sure that Laos and Cambodia is similar to Thailand by using the Buddhist calendar