"THIEF — Mountebank: This sub-class of thief specializes in deception, sleight of hand, persuasion, and a bit of illusion. These factors, together with speed, are what the mountebank relies upon. However, disguise and theatrics also provide valuable tools of the trade to this class, so that one might never know one has been had by this class."
Thus spake E. Gary Gygax in issue number 65 of Dragon magazine, way back in 1982. He was speaking in the context of new player-character classes that were "being considered for inclusion in the expansion volume for the ADVANCED D&D® game system." They were to appear in Dragon prior to the publication of that volume (Unearthed Arcana), but alas only two of the six ever made it into print. The mountebank, jester, savant, and mystic have never seen the light of day. In the latest issue of Footprints (#12), David Prata finished a two-part piece on these classes, and did an admirable job in doing so. However, in the case of the Mountebank, I believe he overlooked a prime source of information.
In EGG's collection of short stories, "Night Arrant", specifically the story "The House in the Tree", we are treated to one of the many memorable characters of the Flanaess; Hop. Owner of the Inn of the Brothers of the One and Score, only a few days' ride from the City of Greyhawk, Hop is explicitly described as a mountebank, and his description in that story (he appears in another, but only peripherally, and with little if no relevance to the subject at hand) lends itself to the description of the intended sub-class as related above.
Hop is described as "charismatic". (I take that to indicate a minimum charisma score.) We are told "If he truly had bardcraft, as some claimed, and some small skills with unusual dweomers, as others claimed, then this man could be the Mountebank of Mountebanks!" So, bardic powers (and bear in mind we're talking about 1E bards here) and a spell list with many unique spells as well, acquired at higher levels (like the Ranger or Paladin, but with new spells).
Hop is said to sell quack medicines for profit, although he is equally said to sell useful ones as well. As Gord put it... "But... you offer spurious cures for the gullible and credulous, not real, working potions! How come I feel so... so lucid?" It is obvious that, although the Mountebank's concoctions are often mere trickery, he has at his command the power to produce ones of real effect as well. Does he even know the difference? Who can say? Look to the purveyors of patent medicines in the late 18th and early 19th century for inspiration on that aspect of the Mountebank's abilities.
It should also be pointed out that the main portion of the story is brought about by Hop's desire to acquire magical mushrooms. He obviously knew of their efficacy (he is able to identify the valuable ones by color), and so no little skill in herbalism and the concoction of really efficacious potions is indicated. Perhaps with a chance to fail, but not always...
What strikes me is one very subtle bit of characterization. Hop always refers to himself (and is called by others) as Hop the Savant. We know from the above that the Savant was to be another character sub-class. In the stories, Hop certainly plays the role well, quoting Yogis from their arcane wisdom, but there is always a tinge of cynicism in his quotations. It's almost like he's giving a credible performance, rather than a true expression of the Savant's calling (although it might well be an insight into how the Savant was originally conceived as well!).
Which leads me to my next and final supposition. Perhaps the Mountebank has within his bag of tricks the ability to imitate other character classes. Within limits? Certainly. But a skill at illusion, disguise, and sleight-of-hand would certainly lend itself to such a power. Imagine a Mountebank posing as a pious Paladin, fleecing his prey and then disappearing into the wide world wondering where that sterling fellow with the fine armor who "spoke" with his horse had gotten to? Just supposition of course, but it certainly fits and wouldn't it be an out-of-the-box (and very useful!) sort of power to possess?
We'll never know what these classes were ever intended to look like, because EGG was very tight-lipped about them, as a result of his falling out with TSR. Short of those manuscripts being published (and I certainly hope for that day!), I hope these poor notes give some inspiration.