The original implementation of awk. Named after the initials of its primary author, Brian W. Kernighan. Bwk awk is still bundled in several operating systems, including FreeBSD?, NetBSD?, OpenBSD? and Darwin (MacOS? X). It is considered the reference implementation and is described in the book "The AWK Programming Language" published in 1988, (ISBN 0-201-07981-X).
The latest source (and Windows binary) is available at http://www.cs.princeton.edu/~bwk/btl.mirror/index.html. Many historical versions are available in the git repository at https://github.com/danfuzz/one-true-awk.