What is the earliest use of the "this" keyword in any programming language?

I understand the this (or self or Me) is used to refer to the current object, and that it is a feature of object-oriented programming languages. The earliest language I could find which has such a concept was Smalltalk, which uses self but was wondering where and when (which programming language) the concept was first implemented?

Simula 67 is generally considered the first object-oriented language and predates Smalltalk by a number of years.

It also used the this keyword for the same concept, which can be seen in this book chapter extract:

class Linker;
  begin
     ref(Linker) Next, Sex, Employment;
     text ID;

     procedure Add_to_List(LHead); name LHead; ref(Linker) LHead;
     begin
        Next :- LHead;
        LHead :- this Linker
     end..of..Add..to..List;

     procedure Onto_Lists(Gender,Occupation);
         name Gender,Occupation;
         ref(Linker) Gender,Occupation;
     begin
        Sex :- Gender;
        Employment :- Occupation;
        Gender :- Occupation :- this Linker
     end..of..Onto..Lists;

     InImage;
     ID :- Copy(SysIn.Image);
     InImage;
  end--of--Linker;

StackExchange Computer Science Q#121528, answer score: 61