Doh - must have been thinking of perl for the eq command.
Looks like eq appeared in 8.4.0 actually - here's the CHANGE file for the release:
http://sourceforge.net/project/shownotes.php?release_id=109968 - came out in Nov 2000, so that's probably why I've seen it around.
8.4.0 I think was only just recently compatible with AOLSserver (3.5.1 in Nov 2002 ?? ), which explains why nobody's really using it - it being [expr eq].
