While digging through Illusionist materials recently, I went back to look at Peter Aronson's original article in SR#4, and the sub-title of the new class stuck out to me:  "ILLUSIONISTS!:  GENERALLY APPEARING AS A NEW CLASS FOR DUNGEONS & DRAGONS" (page 5).  

The "GENERALLY APPEARING" is what really caught my eye, and given the history of the thief class, it made me wonder whether the illusionist had previously appeared in another zine prior to TSR publishing it.  

Some quick googling found this tidbit from Jon Gawne:

The Miniatures Page wrote:I was there for the creation of the Illusionist class. Peter Aronson created them at MITSGS. It was always a pretty cool idea and I think one of the first "official" specialized classes.

MITSGS = MIT Strategic Games Society and some background @ Rewards in Dungeons and Dragons - Story Games

Peter apparently liked PatW, per Peter Aronson (Phoenix, AZ)'s review of Playing at the World: A History of Simulating Wars, People, and Fantastic Adventure from Chess to Role-Playing Games

Anyone know more?