After the first verse, they introduced newly penned part, Peace on Earth, written especially for the duet.
The Christmas special was recorded on September 11, 1977, at Elstree Studios.
Crosby died almost five weeks later, before the show aired on Christmas Eve that year.
For years, Peace on Earth/Little Drummer Boy was available as a bootleg with Heroes but was officially released in 1982 by RCA Records.
source: express.co.uk