Since I've recently implemented a text editor based on a piece table myself, I spent some time researching where the concept was first used.
One of the first references (1974) I found was to the Bravo text editor for the Alto from Xerox PARC.
"Alto User’s Handbook"
http://research.microsoft.com/en-us/um/people/blampson/15a-A...
Then in 1985 a text editor called Lara by J. Gutknecht who in the following years joined Wirth to build Project Oberon whose text management system is also based on a piece table.
"Concepts of the text editor Lara"
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.98....
If somebody as other, in particular earlier references, that would be interesting.
My recent implementation:
https://github.com/martanne/vis/blob/master/text.c
