I never quite understood why IBM stood by and let Lexmark---which was, by then, a separately owned company---run off Model M's (even calling them "Model M" on the label) for non-IBM customers like Dell.
I know you can't copyright a keyboard layout. However, IBM patented the buckling spring switch in 1978, which means the patent was still in effect in '92, when this Dell/Lexmark board was made (patents running 20 years till they must be renewed).
Were mechanical keyboards already on their way out by then, to the extent that IBM didn't care? But IBM was still Lexmark's biggest customer. Why would Lexmark run the risk of alienating them by selling copies of their (IBM's) own keyboard to competing companies? Did they actually phone up IBM and get permission to do it? Did Lexmark pay IBM a royalty on each non-IBM Model M they sold?
The answers to these questions may be lost in the mists of computing history, but I can't help wondering.