Please refer to article KB55446.
While KB55446 does not seem to be available anymore, 55448 does not answer my question.
I'm not looking only for the latest release, but all the ones before as well, as long as it is a signature set version 5.1.
This article is available on Service Portal site -
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KB55446 has a list of all Sigset Releases for 5.1, 6.1 and its corresponding release dates.
My apologies, it seems I really wasn't logged in at the time I checked your first answer.
Thanks for the help, this is what I was looking for !