This is something I've been thinking of, no sources or rumors or anything like that. It stands to reason. In the first season of Sesame Street (1969-1970), Jim Henson, Frank Oz and Caroll Spinney were the only Muppeteers - at least the only Muppeteers who provided their own voices, there were obviously other silent puppeteers. Also, Caroll often performed AM roles & background characters. Like in "Goin' For a Ride". So if in fact he did perform Ernie's right hand it would make sense. As usual, this question is for anyone in the know, or who just likes to make guesses.