@Bart *************
When you ENTER with a 60 days tourist visa, you receive a 60-days stay permit, and the visa becomes invalid for further use.
The 30-days touristic extension is an extension of the stay permit.
It is not an extension of a visa.
The visa is invalid already and thus can't get extended
A visa is a permit to enter.
The length of the stay permit is determined by the visa class
After you entered with a visa, you are staying in Thailand not on a visa, you are staying in Thailand with a stay permit. The stamp you receive upon entering, is an “admitted stay stamp”. This stamp is not a visa
I am fully aware that Immigration in their bad English, don't draw a difference between a stay permit and a visa.
Thai Immigration calls the extension of a stay permit "visa extension".
This is just another example of WRONG use of English and the ignorance for the basics of understanding what they actually are doing.