IMHO, as with any reference book there are usually oversights, omissions or faults. A reference as huge as Mannel's would be no exception. Not to speak badly of Mr. Mannel, I look at it as the bible of Ford small blocks! Keep in mind also that references are written based on the best information available at the time.
With that said, while he lists "551-B" in the tables, if you refer to page 4-8 you'll see your tag bearing 551-C (though minus the "N"), in image "b" of 4A6, as well as some information in the description. He also references the oversight in his update online.
http://www.fordsmallblock.com/changes.htmlBTW, since the tag could be easily transferred from one engine to the next, it's not a good indicator of originality. Checking the assembly date stamped on the block would provide a better clue, as well as casting codes.