You have installed a 14.3 RU5+ SEPM in your environment and found that no 14.3 RU5 version install packages for Linux and Mac are present.
14.3 RU5, 14.3 RU6
These packages are now released and included through LiveUpdate and are no longer bundled directly with the installation.
After a LiveUpdate session has taken place on the SEPM, the packages will populate the most recent supported version.
See Versions, system requirements, release dates, notes, and fixes for Symantec Endpoint Protection and Endpoint Security to determine the build number of your most recent supported client and whether it has released yet.