The short answer is that sometimes you can tell from the ISINs that two bonds have the same issuer; but the converse does not work. Some bonds have a CUSIP assigned (for a small fee when the bond is issued) by the CUSIP authority (which happens to be Standard & Poors). A CUSIP has 9 characters: 6 characters for the issuer; 2 characters for the issue; and ...


The reason the spreads were off is that the data came from MARKIT, and MARKIT often includes a 3M spread (but does not always publish it). So the 3M Quoted Spread and 3M Par Spread are exactly the same (but unfortunately invisible). And therefore Par and Quoted for >3M will not be exactly the same.

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