History of the pullback corner notation Where/when did the convention originate of marking pullback (and/or pushout) squares by that little right-angle symbol in the corner?

The earliest instance I’ve been able to find is in Paul Taylor’s diagrams package, from ≤1994, as mentioned in e.g. the changelog notes for v3.81 at http://www.paultaylor.eu/diagrams/ .  But it seems more likely that this was to meet the demand for a notation that was already established, rather than being the origin?  But looking at various well-known category theory textbooks from before 2000 (Mac Lane Categories for the Working Mathematician; Mac Lane and Moerdijk Sheaves in Geometry and Logic; Borceux Handbook of Categorical Algebra; Johnstone Topos Theory), none of them seem to use it, as far as I can find.
 A: A small comment, that does not fit in its proper place. The department of Algebra in the University of Santiago de Compostela has studied categories since the second half of the sixties under the leadership of prof. Eduardo García-Rodeja. I made my studies later and the following notations were standard:
• Cartesian square:

• coCartesian square:

So, I was not surprised to see Paul Taylor's notation. And for me, too, it was the first time I noticed a notation similar to the one it was used here in print. later, the main research areas where closed categories and homology theories, though after all these years, it has diversified a lot.
A: In an email to me dated 17 February 1992, Peter Freyd said:

I was using a different notation in 1974 in lectures at Montreal. A high school teacher named Butler suggested the right-angle. It was an improvement. I have used it since.

When the diagram gets too complicated for the pullbacks to be rectangles, such as in the final chapter of my book Practical Foundations of Mathematics, I strongly recommend making them at least parallelograms.  Then it is clear that pullback is acting as a functor that transforms one part of the diagram to another. In particular, in a type-theoretic setting pullback is substitution; whilst this has been known for a long time, Section 8.2 of the book actually proves it.
William Butler (had) proved some important results about monads, which you will find in "Toposes, Triples and Theories" by Barr and Wells (free TAC reprints copy).
Other very smart categorists who left academia to become schoolteachers include Christian Mikkelsen (who was the first to derive colimits from limits in an elementary topos) and Sjoerd Crans (who did weak higher dimensional category theory).
A: If it is of any use, on page 251 of Taylor's "Practical Foundations of Mathematics", shortly after introducing pullbacks, he writes:

Pullbacks are often indicated with the right angle symbol, which was suggested by William Butler in 1974 and popularised by Peter Freyd.

