Mongolian Joining Type

[Richard Wordingham]

I've been trying to understand the joining type logic that categorises
a Mongolian letter as isolated, initial, medial or final, and the
consequent effect of free variation selectors.

As far as I can tell, it is currently supposed to be controlled by the
property joining_type. However, this property appears only to have been
non-trivially assigned to the characters of the Mongolian script from
Version 6.3.0.  How was categorisation assigned before then?

I am particularly interested in the intended effects of U+202F NARROW
NO-BREAK SPACE and U+180E MONGOLIAN VOWEL SEPARATOR.  They seem to
presently have a Joining_Type value of Non_Joining, but some things
would make more sense to me if they had the value Dual_Joining.  I am
wondering if their effective value has changed; e.g. previously the
definitions for Mongolian characters worked as though they were dual
joining, but when matters were formalised they accidentally became
non-joining.

Richard.