Taking a quick look online, Key Ovation owns the patents for the split-adjustable platform. Their keyboard, the Gold Touch, is very similar, but they use rubber domes. The sources I found online aren't official, so they could be wrong, but it seems plausible. From the pages I found (essentially blogs and message boards), Key Ovation did the overall design for IBM/Lexmark who then produced the board. When Lexmark stopped making the M, and sold the patents to MaxiSwitch (then Unicomp), they did not get the M15 patents (they were probably just licensed from Key Ovation).
Again, take this with a grain of salt as I can't verify any of it.