Back in 1984 I was 16 and took a summer job at a Marina wrenching on inboard and outboard motors. The boss man had a late 70's Mercury Marquis I think, it was a great big ol' sumbich of a car at any rate was powered by a 351M-400. He always said he had a "400 Michigan" in his car. Which I thought odd and never believed or heard this before or since. What I have heard on numerous occasions is the M stands for a "Modified" 351 Cleveland turned into a 400. Which if true would make Ed's theory the correct one and the 351M-400 moniker make some sense, there only was one motor, the 400.