It seems that while there may not be much in the way of "real" references to any 6 fingered aliens in UFO lore, TV's "The Outer Limits" presented us with their version in season #1, circa September 1963. The episode was called "The Sixth Finger". David McCallum was made up as a hyper-evolved Welsh miner! It's a long story, buy the DVD First Season set and check it out for yourself! In a "one-off" kind of manner, he looks, or has a resemblance to our Santilli creature...
The REAL
DEAL
The Roman Statue
The X-Files take on the Nephilim
Real Human Afflictions and Deformities
...There is also another possibility. There may have been an earlier leadership which was six-fingered. The strange passage in Genesis Chapter 6 indicates that there were - both before the flood of Noah and subsequently - some strange hybrids called, in Hebrew, the Nephilim, which were, among other things, six-fingered...