I am pretty sure they were there a while ago, and, when googling it, there are so many posts mentioned these three master pages. Now, what we have in "Publishing Portal" site is below:
Adrian noticed this issue in the post , but didn't provide the reason. Considering the publish time of that post, I assume the change was made by SharePoint 2013 Public Update March 2013. However, Microsoft didn't mention this change in Description of the SharePoint Foundation 2013 update: March 12, 2013 and Description of the SharePoint Server 2013 update: March 12, 2013
In my opinion, use "seattle.master" and "oslo.master" to replace "v15.master" "belltown.master" "overlay.master" is good. It's more reasonable.
Anyway, this is the reality and we have to accept it. :-)
Below are two screenshots to compare the major differences.