I came in with using my social security letter for the monthly. You are allowed to use it for the 1st year, you do not have to wait and only use it for the second extension.
Since you are USA, if you are using Social Security retirement, you will get the documentation letter confirming your retirement amount, which shows your monthly. If you are meeting it with other pensions/etc. then they need to be shown that they come in monthly above 65,000. This is filed with you application for O-A retirement.