What stamp will I receive when re-entering Thailand with a new passport while having an extension of stay in my old passport?

Yep, it will be until your current extension expires

All it will be is just the small rectangle entry stamp showing the date you arrived and the date you're allowed to stay (the date your current extension/re-entry in the old passport expire).

They will note on the new passport that your extension/re-entry was in an old passport..

Then you have to take the passports to the immigration office where you got the extension and get the extension/re-entry stamps transferred. The airport doesn't do that