What this part is — and what it isn't
The body is molded in black polypropylene (PP), a lightweight thermoplastic that resists common industrial chemicals and solvents but isn't rated for continuous UV exposure. A lanyard is attached so the cap stays tethered to the adapter when removed, which is handy on a test bench or in a rack where small parts vanish fast.
That means TE Connectivity no longer manufactures the 221586-1, and there is no direct factory replacement listed. No official cross-reference or successor part number has been published, so any substitute must be evaluated on fit: it must snap onto a BNC Tee adapter and clear the adjacent ports.
Fit check — what it mates with
The 221586-1 is designed specifically for use with BNC Tee adapters — it caps the open female port on the tee. The polypropylene body snaps over the BNC bayonet interface and is held in place by friction and the lanyard keeps it from walking off. It does not mate with standard BNC plugs or jacks; it's a protective cover, not a connector half.