Fits the profile and the reports of prior use of SBTC signature on notes/letters to his first child bride. I believe the ransom note was written to fit into his "script" of his perverse fantasy, rather than a legitimate ransom note.
There is NO possible way he would know about the $118,000 AFTER TAXES bonus that Jonbenet's father got that year and that note doesn't fit into any fantasy of his at all. And now, the handwriting doesn't match. The three letters that sort of match ( I don't think that they match at all! ), aren't enough letters to make a legal match, in any court.
Fits WHAT profile? Karr hasn't ever gone after a little girl he never even ever saw, let alone met. And he NEVER met nor saw Jonbenet until her murder was splashed all over the media.
SBTC -- in the yearbook: Shall Be The Conqueror