I did some more digging, starting off where Mark has kindly pointer me in the direction to look.

The .spb file extension is a saved game file created by Microsoft ESP, a software toolkit used for flight and vehicle simulation.

It saves the state of a flight simulator mission, including the state of mission "triggers" and "actions" and is saved with a .FLT file as part of a game save.

Microsoft ESP missions are originally created in an XML format, but are saved in a binary SPB format to speed reading in the data when FSX starts up.

I had to use an FSX SDK tool to reverse engineer it back into an .xml file. But I'm not sure of all the xml tags so I don't know if this is the file I need to edit or not. I will have to keep researching this it looks like.

In any event, here is a link to download the xml file since it won't post here for security reasons.

http://www.SimChecklist.net/icons.xml