
Outside RCA Nashville's "Studio B" in May 1971
Source: http://www.elvis-collectors.com/candid-central/studio71.html
I just found today that someone uploaded the complete jam of "Merry Christmas, Baby" on YouTube.
It is the full, undubbed and unedited take, plus a bit of Elvis instructing the rhythm section before they begin.
So it was kind of a "planned jam" after all. It appeared on an excellent Venus CD release a few years ago, one which is still worth seeking out, if you are another crazy fan. I know I am.

http://www.elvisinnorway.no/uneditedmasters70rev.html
Enjoy!
..
Elvis Presley "Merry Christmas, Baby" (RCA Nashville, Saturday, May 15, 1971)
Track recorded in one take ... "Wake up, Put!"
https://www.youtube.com/watch?v=v35LZU6xkGg&t=34s
Note that when RCA released "Merry Christmas, Baby" in 1971, it had stinging lead guitar overdubbed by Eddie Hinton, which I really liked. I still prefer the album cut best, and Hinton's work is one of the reasons why. Plus, the full-length version does seem to go on just a ... tad too long.
Remember, longer is not always better.
Ho, ho, ho!

BONUS ELVIS
In Vernon's office at Graceland - Monday, March 7, 1960
"I'm dreaming ... of a white CHRIS-MUSS ... just like the ones I used to know ..."