Following on from an earlier post "The market data product that the global financial services industry really wants..." I was having a debate with an old chum. He suggested that I was mistaken and that in the exchange traded equity world all you need is ISIN+MIC to identify any instrument.
If you open another browser window and hit http://www.londonstockexchange.com you can enter that ISIN into the search box.
So, for one ISIN on one Stock Exchange with one MIC (XLON) we see that there are two Symbols. CSP1 has a Sedol of B521C87 and CSPX has a Sedol of B50YWZ5. The material difference is that CSPX is denominated in USD and CSP1 is denominated in GBP.
CSP1 is listed on Bloomberg as CSP1 LN and Reuters as CSP1.L
CSPX as CSPX LN and CSPX.L
So, if you are going to send a FIX order to a broker and just send ISIN and MIC you really also need tag 15 (Currency) to be sent as well. If you are interfacing with other systems you will need to ensure that Currency is included in your call to that system to ensure unique instrument identification.
The devil is in the details and instrument symbology is all about details...