Do you think it would be possible to use an old embassy letter to get an extension towards end of September.

 

I asked for one the last time but didn't need to use it in the end.

 

It's very generic and just says when my last visa ran out and asks if they can please give me an extension.

You need the embassy letter to show evidence for a "case of necessity" at the time you apply for the extension. In other words, immigration places the onus on your embassy to verify and confirm that you have until that date tried and been unsuccessful in finding transportation to leave Thailand.

The letter most likely has a date on it, and if they request a 30 day extension in May...

I don't know anybody who tried to use such an old letter, but if you are going to do it, the chance to have success is probably very close to zero.

If you try it don't forget to share your results.

As always, a normal assumption would be that is the document has a date when it was issued on it. Then you can not use it if it´s older then 90 days.

 

Can even be shorter time regarding this kind of letter.

6 hours ago, finy said:

Do you think it would be possible to use an old embassy letter to get an extension towards end of September.

You will certainly need a new one before you can use it a apply for an extension.

I would say a few weeks would be the longest they would accept one.

Khon Kaen Immigration told me that the letter is only valid for a week.  You will get 30 days from the time you turn up at the immigration office and not from the date of the expiry of permission of stay.  Hence the reason the immigration bloke told me to come back nearer my expiry of permission of stay and he saved me 1900Baht as the amnesty kicked in a week later.

At the risk of being sarcastic, you can only use an old letter to apply for an old visa!  If it has an old date on it you will be denied, as others have pointed out in their comments. And you might suffer in some worse way if you try to fool them in such a way.

if its anything like this your OK
Generic not date stamped

6 months is the normal time of validity for an income letter, or it used to be prior to 2019. There's been reports early last year about people from the US about the 6 months validity. 

12 minutes ago, Max69xl said:

6 months is the normal time of validity for an income letter, or it used to be prior to 2019. There's been reports early last year about people from the US about the 6 months validity. 

He is asking about a letter requesting 30 day extension not income proof. I would say a few weeks at the most for them.