On the TM8 form (Re-Entry Permit), at the backside, is the following field:

My previous visa for Thailand is of the category,

 

 Non-Immigrant Visa, Tourist Visa, Transit Visa, Others

 

Do I have to enter my last retirement extension date (last week), or the first original visa (in my case None-Immigrant, 8 years old)

I have spoken to people that have put either one.

I put my most recent permission of stay stamp date. In any event the io will pick it up if not happy.

I put Non Imm and is always okay, after all they are stating “visa” .

You should state Non O as the original Visa used to enter Thailand.

You then state the place and date of issue of said Visa.

The date you originally arrived in Thailand on the Non O.

Finally the current permission to stay date.

Personally, over the last 7 years I put an "X" in the Non-Imm box and then just leave those other fields blank and let the IO fill them in. Never been an issue.

 

I put the date of my most recent entry, in my case 5 July 2019, and not the date I first arrived on the original non-Imm B.

 

I guess if asked I'd say I wasn't sure what they were looking for, and didn't want to make a mistake.