This Westinghouse Desilu Playhouse: Desilu Revue Holiday Special from 1959 is the last missing special featuring the I Love Lucy cast. It only aired once in 1959.

The last time parts of the special was online was 20 years ago with the birth of the web on a site called TVParty.com http://www.tvparty.com

It's finally available, and looks to be in it's entirety...

https://www.youtube.com/watch?v=JEiqmzeSZEU&feature=youtu.be

They made 2 Westinghouse specials.
Here is the first...
https://www.youtube.com/watch?v=TIPkhqhnwho
This was colorized about 20 years ago.

Here are some of the commercials they filmed for Lucy-Desi Comedy Hours...
https://archive.org/details/ILoveLucyWestinghouseCommercials