I recently bought a used zen that was advertised as a SE84UFO.  The amp looks exactly like the SE84UFO that I bought direct from Decware in 2016.  Anyway, after looking on the bottom, I find it is a SE84CKC made in 2015. Is this the same amp, but numbered differently?  A prototype model?

The difference should be the output transformers. . . the CKC was "pre-UFO transformers." In June 2015 the CKC become the UFO model. If I remember correctly there were a few that were on order that were CKC when the UFO changes were to be initiated that Steve offered to make the UFO model. That may be the case with this one? There may be a post in the appropriate forum if that is the case. If not then if it's a CKC it does not have the UFO transformers.

Here is what the product page says:

SE84CKC / Manufactured from 2013 to 2015.  Known as the "SuperZen" the chassis has been improved again with a 1930's typewriter finish. By far the best sounding version to date due to a re-voiced front end. Each amp now comes standard with cryo treated beeswax caps. We named this update the CKC mod which stands for "Christmas Keeps Coming" which can be seen in the model number.
     
SE84UFO / JUNE 2015. Taking everything we've done to date the only possible thing that could be improved is the actual thing that made the amp sound so much better than it's competition... the output transformers. Decware SET output transformers set this amp apart with the ability to drive a dead short, and maintain flat response out to 20kHz. Now with our upgraded UFO wide bandwidth transformer standard equipment, it makes this amplifier simply untouchable.