51-6 is the 6th build in week 51, 51-7 is the 7th build in week 51. The difference between the two builds is likely to be incredibly minor, and obviously not worth making available for download. Go with 51-6, it's fine.

As for your reboot loop, sounds like you need to disable lifeguard-reset - details should be in the newsgroup post you reference. You'll need a Linux PC and flasher-3.0 executable to disable the lifeguard-reset.