My 1520.3 is running 8.1 Update + Denim. (8.10.14234.375) It has never been on Insider or Preview before that. Always official. A manual check showed no updates.
WPRT updated itself to 3.1.5 for me today. The changelog over 3.1.4 showed only bug fixes. I plugged into USB and see this:
I made my backup and installed the update. It was a 1.62GB package.
The update forced me into setup when the phone rebooted.
motion data,
touch and
call + SMS filter all received updates. I can tell because I have the old school Settings menu where it is all one column and the most recently updated items fall to the bottom of the list. I had an OTA Hey Cortana update 2 days ago and now the 3 listed are beneath that.
In about, I see the same OS version number as before (8.10.14234.375) and the promised new Firmware revision number shown in the pic above.