I think that the "O" is supposed to be a symbol for the One Ring, thereby labelling the card a "Ring-foil" which is a masterworks of legendary proportions, hence "Legends Masterworks".
And 9 Masterworks were issued per set in allusion to the 9 rings of power given to men, meaning that we - as the Kings in the books eventually succumbed to the rings - would eventually succumb to the influence of the cards by spending way too much money on the TCG and becoming "card wraiths" ourselves

Cheers!