Reverse lookup: funds holding a security

The reverse index over the whole N-PORT holdings corpus: give it a security, get the funds that hold it. The default view answers "who holds it NOW" — for each fund, the position from its latest published reporting month that contains the CUSIP — while period= answers "who held it in month X" for any historical month.

Each row is one fund (series; whole-trust filers key by registrant so distinct trusts never merge; a trust that received its series id partway through is one fund, its earlier reports without one attributed to the series) with the position's size three ways: balance in the filed unit, val_usd, and pct_val of the fund's net assets — with fund_net_assets on the row so concentration is readable without a second call.

Scope note: this is the REGISTERED FUND ownership map (mutual funds + ETFs from their monthly N-PORT reports). It complements, and does not replace, the 13F institutional view: N-PORT covers bonds and derivatives position-by-position and reports monthly, while 13F covers all large institutional managers (including hedge funds, which file no N-PORT) quarterly on equities. For the 13F-side reverse lookup use POST /api/v1/form-13f/holdings in the Institutional Holdings group.