Below are sources stating that the monthly MBS purchases by the reserve each month are NOT government guaranteed.
Guaranteed by Fannie Mae, Freddie Mac, and Ginnie Mae. The current face value shown is the remaining principal balance of the securities.
Also found in the prospectus for Fannie Mae:
Fannie Mae Guaranty
We guarantee to each trust that we will supplement amounts received by the trust as required to permit timely payments of principal and interest on the certificates. We alone are responsible for making payments under our guaranty. The certificates and payments of principal and interest on the certificates are not guaranteed by the United States and do not constitute a debt or obligation of the United States or any of its agencies or instrumentalities other than Fannie Mae.