I saw this on kbtalking while looking for information about the mystery keyboard ligo posted:
http://kbtalking.cool3c.com/article/23272
1990 IBM Model 300, Part # 6019303, POS terminal keyboard, made in Armonk, NY. Is it a buckling spring keyboard?
Show Image
(http://geekhack.org/attachment.php?attachmentid=10637&stc=1&d=1275718333)
Edit: Uploaded one of the pictures from there so that you don't have to wait for that page to load if you are on a slow connection.
Wow, that page is slooooooooow...
If it wasn't for the stamp on the plug and the label I'd say it's a clone. Although the keycaps are almost certainly IBM ones.
Just a few observations -
* In the 80s, it was common for IBM to specify the model of the computer it was designed to be connected to on the keyboard. This is why the 56409xx keyboards are labeled "Model C1/C2/C3/C4" depending on example - they were not Model C keyboards, they were the Model F keyboards of the Model C1(etc) terminals that they were designed to be connected to. Judging by the shape of the case, this keyboard looks like a Model F.
* The labeling is misleading - Armonk is the location of IBM's corporate headquaters, this would have been made in their plant in Kentucky.
I might have more observations when the page from Korea loads next year.