@Gábor *********
that's not correct at all.
A visa is a VISA. That is used for entering Thailand.
An extension is an extension, it does not have a visa type because it's NOT visa. The stamp in the passport says "extension of stay" because it's not a visa.
You can apply for an extension of a non-OA visa based on being over 50 (retirement). You can apply for an extension of a non-O visa based on being over 50 (retirement). Once you're on an extension you're not on any visa. You're on an extension based on a specific reason.