Galler, B.A. et al, "A Proposal for Definitions in ALGOL 1969"

[ACM] (1967) [ACM] CACM 10(04) (April 1967).  

Languages mentioned ALGOL C {1966} ALGOL 60 with extensions to permit user defined extensions ALGOL D {1969} Hypothetical family of syntax-extended Algol 60s