Will return to Thailand on my retirement extension soon and wonder what I should write in the "Visa number" column as there is no number on the extension stamp. Do I write the original visa number the extension is based on or the number of the re-entry stamp?

I fill in the number of the re-entry permit and write "re-entry" next to the box.

 

You should write the re-entry permit number, and make sure the IO sees you have a re-entry permit and uses it.