Hi Suresh
These both are patch 1 for 15.5.0:
Version/Build 15.5.0.225 01_PRE 6
Version/Build 15.5.0.225 01 19
First is pre-release, build 6. Second is the released patch, build 19. If you think between both, install the latest one.
15.5.1 is not a patch, but Service Pack. It's downloadable from our site as a separate Release.
Hope this helps -Nika