Earlier quoted context omitted.
Isn't the Unicode codepoint repertoire pretty much identical to ISO 10646? AIUI Unicode only differs by standardizing additional character properties and rulesets, but the encodings are supposed to be identical.
They weren't using Unicode at all. Instead they were using 'prehistoric' fonts patched by changing the glyphs in ASCII fonts(100s of them). This too without proper conventions. They way to convert these to Unicode is by creating character maps in a font-by-font fashion.
ISO 10646 does not work as you describe. I'm pretty sure most of the character encodings defined by creating a custom font were never standardized by ISO.