There was no ISO file released for QRadar 7.3.1 Patch 8. If you need to complete a QRadar reinstall, you need to install the 7.3.1 Patch 7 ISO file, then apply the 7.3.1 Patch 8 SFS.
There is a note on the master software page
https://ibm.biz/qradarsoftware for 7.3.1 Patch 8 that mentions that this release is an SFS only.
If you have follow-up questions, let us know.
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Jonathan Pechta
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