Where is the original literature for the specification of an Associative Array ADT?
The consensus from the available information I've found points to SNOBOL4, released in 1967, as the earliest implementation of "associative arrays".

This post on the Software Engineering community is very similar to your question and has some great info.


Chicago University - Intro to TCL course: Associative Arrays



  Associative arrays were first used in the programming language Snobol4 in the mid-1960s. They are a very common data structure in the Unix environment, being provided as the major data structure by languages like Awk, Perl, etc.