In another question where EU2 was behaving differently to EU5 I commented:
For what its worth, trust is listing EU2 as "Release: Summer '15 Patch 9.6" and EU5 as "Release: Summer '15 Patch 10". NA1 is on 9.6. I can't find anything that would explain the differences between 9.6 and 10.
Which left me wondering, How can we tell what is changing with each patch release?
The triannual Summer, Spring and Winter major releases are well documented.
Where is the documentation for the contents of each intermediate patch release?
@TimChadwick helpfully pointed me to https://success.salesforce.com/issues_releases_view?release=190000000, which shows the list of main pods and their current patch status.
What I found more interesting was the release=190000000 in the query string.
196010000Summer '15 Patch 10.0
194012000Spring '15 Patch 12.0
192013000Winter '15 Patch 13.0
This certainly seems useful, but applying it to the Summer'15 Patch 9.6, which I know existed as I noted it in the question, didn't reveal any known issues.
Also, not everything that is being patched appears as a known issue. Is there any other source of changes in the patches?