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Okay so I found more details here and I learned 3 things:
  1. The holdings are indeed all F/GNMA
  2. The MSBs get revalued every week
Then I downloaded a CSV for MSBs with the form at the bottom of that page with 6/18 and 6/25 data... and these seem to have lost value for...
17,638,065,700.96 = 17B in a week.
Now what I don't know yet is whether this is revaluation or partial sale. That's something for later