[LEAPSECS] ISO 8601 Z designator improper before 1972?

[Gerard Ashton]

Given that UTC did not exist in its present form (and as described
in ISO 8601:2004, that is, with leap seconds) until 1 January 1972,

and

given that the ISO 8601:2004 standard states "[Z] is used as UTC
designator."
(p. 12)

is it fair to conclude that it is improper to use the "Z" designator for any
date/time in the ISO 8601 format prior to 1 January 1972?

If indeed that is improper, would it not be incumbent on anyone who wishes
to express a UT time/date in a format that looks like ISO 8601 format to
either

(1) leave off the designator, and communicate the fact that the it is UT
through
some other means, or,

(2) establish their own standard that expressly modifies ISO 8601 to
indicate
that "Z" is the UTC designator after 1 January 1972, and the UT designatior
before
then?

Gerry Ashton