Considering how many articles have been written about it, how many toy sales generated, and what an enduring icon of the show it remains, I'm fascinated by how little ever seems to have come up in the show itself.
The best I've found is a couple of lines in Brain Killer, where Napoleon makes a (very) passing reference to it having an automatic mode when lending one to Waverly's doctor. The fact UNCLE agents sometimes use sleep darts in place of regular bullets comes up explicitly or implicitly once in a while (eg, Quadripartite, Secret Sceptre), but other than that, I'm drawing a blank. A search for uses of any variant on 'UNCLE Special' and other various keywords in the subtitle files has found me nothing to add to the above, but the subs aren't always perfect, so there could always be something I've missed. Anyone?
Bonus question: Taking the extreme canon-purist line that only what makes it into the episodes counts, could an argument be made that almost everything we know about the UNCLE Special - including the name itself - is pseudo-canonical at best? *
* No insult intended to the venerable UNCLE Special, I'm just bemused by the possibility.