Build 10586 for PC represents the RTM build for Windows 10's Fall Update, so it makes sense that the same build number might be used for the final build for Windows 10 Mobile's launch. At the very least, build 10586 for mobile will give us a near-complete look at what Windows 10 Mobile will look like at launch.
While Microsoft hasn't yet pushed said build to Insiders, some users on Twitter have leaked a PC emulator for the build. This will allow you to test the build on your PC, and see what Microsoft's mobile OS may look like at launch.
Final bit, 10586 Mobile Emulator https://t.co/0VPZWT6aEB— Core (@tfwboredom) November 10, 2015
Though there seem to be no new features compared to the latest mobile build available to the Fast Ring, build 10581, this is to be expected as Microsoft's engineers will now be focusing on last-minute optimisations and bug fixes to allow for a smooth launch.
As we have previously reported, a Microsoft spokesperson has suggested a December launch for Windows 10 Mobile. Insiders, on the other hand, may see the RTM build for testing earlier.