This is a 1993 Model M (1391401) with the 5' detachable PS/2 cord.
From Wikipedia: http://en.wikipedia.org/wiki/Model_
"The Model M keyboard is a class of computer keyboards manufactured by IBM, Lexmark and Unicomp, starting in 1985. The many different variations of the keyboard have their own distinct characteristics, with the vast majority having a buckling spring key design and many having fully swappable keycaps. Model M keyboards have been prized by computer enthusiasts and heavy typists because of the tactile and auditory feedback resulting from a keystroke.
The Model M is also regarded as an extremely durable piece of hardware. Many units manufactured since the mid 1980s are still in use today, while the computers and monitors of the day are obsolete. Unicomp, which now owns the rights to the design, now sells the keyboards. Recently, the keyboards have made a comeback amongst writers and computer techs. Unicomp has had difficulty making them profitable because they rarely break, and at US$69, the chain retailers will not stock them."
This keyboard is in perfect shape, with one red pen mark on the underside (see photos).
A few more pics can be viewed on Flickr.
Please e-mail zannerat at gmail dot com if interested.