It has occurred to me since writing the above answer to the question about Montresor's coat of arms that the one he describes as "A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel" may be completely fictitious. Montresor has not been telling Fortunato the truth since he met him in the street. Why should we assume he is telling him the truth now? He may be taking a malicious pleasure in describing a coat of arms that would be totally appropriate for what he intends to do to Fortunato and why he intends to do it. I suspect that there is something odd about the motto, too, and something a little odd about Fortunato's response when Montresor tells him the motto is "Nemo me impune lacessit." Fortunato simply says, "Good!" Chances are that Fortunato doesn't understand a word of Latin, although the intelligent reader can guess, even without knowing the language, that it means something like "Nobody injures me with impunity." Montresor is enjoying fooling the wine-befuddled Fortunato and telling him to his face with the description of a false coat of arms and a motto in a language his victim doesn't understand that he is an extremely vindictive and dangerous man to treat disrespectfully.
The word, or two words, d'or mean in gold. In French the word de becomes d' when it appears before a word beginning with a vowel. So in this case it would be pronounced like our word "door." Poe seems to want to emphasize that Montresor is French and not Italian. His name in French means "my treasure." Poe's purpose seems to be to suggest that Montresor remains an outsider, even though his part of the family must have lived in Italy for many generations, judging from all the bones in his catacombs. The fact that he is an outsider would suggest one of the reasons why he feels he has suffered a "thousand wrongs" at the hands of Fortunato (who certainly has an Italian name). Evidently they both deal in paintings, jewels, and wines (with Austrian and British millionaires). Fortunato has more money and can make better wholesale purchases, beating Montresor out of advantageous deals with foreigners which Montresor badly needs; and Fortunato has better connections with wealthy Italians, so he gets more referrals to sellers and buyers. He and Montresor are competitors. Luchesi is another competitor. Fortunato's interest in the Amontillado is purely commercial. Sherry is a sipping wine, and Fortunato guzzles whole bottles of table wine with a lower alcoholic content. He believes that poor Montresor could only afford to buy one cask, but he can buy a whole shipload and resell it at a profit, if it is the genuine Amontillado. The fact that the foot was done "in gold" may also be intended to suggest that the Montresors were a very wealthy as well as a very proud family.