I don't think they're stock cables. The AT-DIN cables on the 1985-1987 Model M's were black, while these are identical to the 1987-1995 cables, except for the plug.
I think the story behind these is that Lexmark offered AT-DIN cables with their keyboards. A few M5-2 and M13 keyboards have been known to come with an AT/Serial cable. It's probably a similar situation here.