It's actually verification of your Thai address by your embassy.
Currently UK,USA,AU don't provide the "letter" (I'm not referring to income letter by same countries for extensions)
An alternative is to obtain a certificate of residence from your immigration office.
That would be impossible at CW as for COR they aso required a TM47 to have been made.
For folk looking to transition to a Non O an agent is an option.
If still in home country then obtaining a Non O prior to entering Thailand would enable you to open a Bank Account.
Bangkok Bank and Kasikorn good option especially if you plan to use WISE.