According to footnote 5 of the NCUSIF's audited financial statements,
"[a]s of December 31, 2012, the NCUSIF had an outstanding collateralized senior note due from an insured credit union for $179.3 million. Accrued interest on the notes is due on a monthly basis. Interest on this note has variable terms."
This note first appeared in the financial statements of the NCUSIF in August of 2012.
The most likely candidate is Evangelical Christian Credit Union of Brea, California.
As of December 2012, Evangelical Christian reported $179,254,167 of outstanding term borrowings from corporate credit unions. This is exactly equal to the amount of the note receivable minus the $70 million in capital notes provided to Texans Credit Union and AEA Federal Credit Union by the NCUSIF.
I suspect that this collateralized senior note had to deal with the 2012 liquidation of a corporate credit union.