While The Shining doesn't have any M's in it, there are words within the 121 pages that may contain a lot of M's, as well as that computer looks like it has received a lot of wear and tear over years of John's career. I think that whole letter thing is a carny trick for him to mess with us.
The codenames this year all seem like they could be very obvious, but without hints we have no way of narrowing it down until a lineup is assumably leaked again