Ok so I really do think Tesler may have invented the keyword for lisp. Lisp70 was based on exploration from MLisp and MLisp2 - neither of which had colon prefixed symbols. I wonder if it was inspired by the smalltalk selector syntax but in reverse? He was definitely involved with that and even further Alan Kay is thanked in the Lisp70 paper. Anyway I would be glad to be wrong if someone can point to prior art!