Swan Song Inc. is the now defunct record label launched by Led Zeppelin on May 10, 1974. Here is the Swan Song Inc. logo directly from their letterhead.
The Swan Song company logo was made by designers Hipgnosis and based on this 1869 painting by painter William Rimmer called “Evening” or “The Fall of Day” featuring a picture of the Greek god Apollo.
Because the logo appeared on the Led Zeppelin “Physical Graffiti” album, many people have confused it as a Led Zeppelin logo.
The logo appeared on all Swan Song releases not just Led Zeppelin albums. Check out the logo on the Bad Company self-titled debut album released by Swan Song.
Swan Song had both a UK and US addresses.
UK Address: 484 Kings Road, London, SW10 UK (John Paul Jones Travel Case)
Swan Song ceased active operations in 1983, and now exists only to reissue previously released material but the logo lives on. It still shows up on Led Zeppelin hats, shirts and memorabilia all of the time.
UPDATE: On June 29, 2015 Led Zeppelin posted this photo that looks like a proof of the Swan Song logo on their official Facebook page.
On my photo website I have my original Swan Song letter for those who wish to see it. Enjoy!