@Nongnuch *******
The misunderstanding comes that you rely upon an English text instead of the actual Thai written law. I rely upon the actual Thai language written law.
1656 precedes 1657, 1658,... but doesn't "govern" or define 1657, ...
There are 5 different types of will
Section 1656 describes the first type of will, commonly known as พินัยกรรมแบบธรรมดา Here the testator can sign or even thumb print a printed will, with 2 witnesses.
Then section 1657 describes the second type of will, commonly known as พินัยกรรมแบบเขียนเองทั้งฉบับ --> this is the handwritten will we're talking about, thimbprint is not allowed. (Wouldn't make sense since this has to be completely handwritten by the testator)
These first 2 wills don't need registering. And the handwritten type (2nd type) doesn't need witnesses either.
Then there are 3 more types :
3.พินัยกรรมทำเป็นเอกสารฝ่ายเมือง --> 1658
4.พินัยกรรมทำเป็นเอกสารลับ --> 1660
5.พินัยกรรมทำด้วยวาจา --> 1663
You can't rely on English versions of the law you find on the internet. The English version is just a "more or less" translation with mistakes.