GCC 6.3 vs 6.3.1 - whats the difference? Is 6.3.1 an unofficial secret GCC release?

[info rmer]

A lot of Linux distributions have gcc 6.3.1 available, or even preinstalled.
However, I am not able to find any info about 6.3.1 at the official GCC pages 
at gnu org.
Please tell, is 6.3.1 an unofficial secret GCC release?
What is the difference between 6.3 and 6.3.1, any changelog?