'88-'91, ...
See Tcl chronology for how Tk fits into things.
In 1991, JO presents Tk at the X Technical Conference and at the Winter USENIX conference.
ftp://tcl.activestate.com/pub/tcl/doc/tkUsenix91.ps and ftp://tcl.activestate.com/pub/tcl/doc/tkF10.ps are the original paper, along with a figure, from that USENIX conference.
[What details should go in here?]
[What and when was the first Tk extension?]
Changes in Tcl/Tk points to a series of pages that detail what features are added, modified, or deleted in each release.