We will be making the current weekly version (3.39) the new quarterly version on or shortly after February 18th, 2020. The versions will then be:
3.37 will be deprecated and you will be served an existing version if you try to request it.
If you don't specify a version, you will be served a default version, which depending on your key configuration will be either 3.39 or 3.40. But a developer specified version will always be honored and served when requested when it's an existing version.
To report issues with any version of the API, see:https://issuetracker.google.com/savedsearches/558438