Early dialect of Smalltalk Language, and considered to be the first "usable" version of Smalltalk (Smalltalk-71 was a rather primitive prototype). For a long time, Smalltalk-72 was a project internal to Xerox Parc, and didn't became known outside until well after Smalltalk-80 (what is usually meant by just "Smalltalk") was released. Smalltalk-72 was described by Alan Kay in Early History Of Smalltalk; and Dan Ingalls has recently made a re-implementation of Smalltalk-72 in Squeak Smalltalk: wiki.squeak.org 
The Smalltalk-72 Instruction Manual from 1976: www.bitsavers.org
See original on c2.com