A particular application or system software component can be comprised of many individual files. A version number is assigned to the whole group of files. Often times, only a few of the individual components are revised, though the version number for the group of files must be revised.
In the case of the Express Modem build, if you open the Express Modem control panel, it will display version 1.5.3. But if you were to perform a "Get Info" on the Express Modem control panel from the Finder, you will notice that the 1.5.3 control panel is part of the Express Modem Software 1.5.4 build.
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