attribute appearance

When did attributes appear in SGML (and its predecessors)?

Charles Goldfarb has an interesting piece of markup at:
http://www.sgmlsource.com/history/roots.htm

It includes this nice bit of shockingly readable but attribute-free DCF
GML:

:h1.Chapter 1:  Introduction
:p.GML supported hierarchical containers, such as
:ol
:li.Ordered lists (like this one),
:li.Unordered lists, and
:li.Definition lists
:eol.
as well as simple structures.
:p.Markup minimization (later generalized and formalized in SGML),
allowed the end-tags to be omitted for the "h1" and "p" elements.


The SGML Handbook (wisely) doesn't explain much about the history of
markup components, but if anyone has a moment to explain this bit of
history, I'd be curious.

-- 
Simon St.Laurent
Ring around the content, a pocket full of brackets
Errors, errors, all fall down!
http://simonstl.com -- http://monasticxml.org