The Dewar story is not about indentation, but about the invention of the colon.
First about indentation in B. Already B0, the first iteration in the B0, B1, B2, ... sequence of designs leading to ABC, had non-optional indentation for grouping, supplemented by enclosing the group between BEGIN and END delimiters. This can be seen in [GM76], section 4.1 (Layout). The indentation was supposed to be added, like pretty printing, by a dedicated B editor, and the user had no control over this; they were not supposed to be able to turn this off or otherwise modify the indentation regime.
Given the mandatory indentation, separate BEGIN and END delimiters are of course superfluous; in B1 we had no BEGIN, but only END IF, END FOR, and so on, and then the latter delimiters were also dropped in B2, leaving pure indentation as the sole indicator of grouping. See [ME81], Section 4 (STATEMENT SYNTAX).
The origin of indentation in ABC is thus simply the desire to have the program text look neat and be suggestive of the meaning, codifying what was already common practice but not enforced. The decision to remove the noise of BEGIN ... END may have been influenced by [PL75], which actually advocated using pure indentation for grouping. Since occam came later (1983), the feature can't have been copied from that language. Same for Miranda (1985). As far as I am aware, B was the first actually published (and implemented) language to use indentation for grouping.
Now the Dewar story, which is how I got the idea of the colon, as I wrote it down in a memoir of the ABC design rationale:
And here I will paraphrase, at Lambert's request.
In 1978, in a design session in a mansion in Jabłonna (Poland), Robert Dewar, Peter King, Jack Schwartz and Lambert were comparing various alternative proposed syntaxes for B, by comparing (buggy) bubble sort implementations written down in each alternative. Since they couldn't agree, Robert Dewar's wife was called from her room and asked for her opinion, like a modern-day Paris asked to compare the beauty of Hera, Athena, and Aphrodite. But after the first version was explained to her, she remarked: "You mean, in the line where it says: 'FOR i ... ', that it has to be done for the lines that follow; not just for that line?!" And here the scientists realized that the misunderstanding would have been avoided if there had been a colon at the end of that line.
Lambert also included the following useful references:
[PL75] P. J. Plauger. Signal and noise in programming language. In J. D. White, editor, Proc. ACM Annual Conference 1975, page 216. ACM, 1975.
[GM76] Leo Geurts and Lambert Meertens. Designing a beginners' programming language. In S.A. Schuman, editor, New Directions in Algorithmic Languages 1975, pages 1–18. IRIA, Rocquencourt, 1976.
[ME81] Lambert Meertens. Issues in the design of a beginners' programming language. In J.W. de Bakker and J.C. van Vliet, editors, Algorithmic Languages, pages 167–184. North-Holland Publishing Company, Amsterdam, 1981.