While working on the recent changes to historical timestamps for Sierra Leone, the unique -11:20 offset for Niue caught my eye in contemporaneous editions of The Air Almanac which P Chan had sourced: specifically, that it was no longer there by the late 1960s. FInding a few more editions, I eventually found -11:20 listed for Niue as of Apr 1964 but -11 as of Aug 1964:
This points to a likely transition sometime around July 1964. 1978 did seem awfully late for such a unique offset.
Proposed patch attached, and installed in the development repository.