Last October in Khon Kaen I used income letter for the first time.
Attached to the letter were two pension statements (in German language).
I explained what it is and officer was OK with it.
(I had prepared tons of copies about source and transfers also, unused)
Somewhat weird that you also had to show amount of transfers etc.?
I never care about these extra documents aside from TM7.
As you mention they can change any other year.
It's just filling personal data and signing. Done quickly at the office.
Little off topic:
For next extension I asked at KK about the 12 monthly transfer method.
It's available for ALL nationals whether or not they can get an income letter.
Besides the known requirements (bank book, bank letter, 12 month statement) I should also show some proof of income source (similar as for income statement).