I think Upgrade Advisor enters you at 10.0.10586.127 and you then use the normal update check to bump up to .420...
Release Ring is 10586.420 after all updates are sought/found
Slow Ring is 14367.x (the last digits change frequently)
Fast is 14372.x
Any/all of these can shift at any time, even Official/Production. WaaS brings new updates to official normally once a month on Patch Tuesday, but if some patch or fix becomes urgent/vital it will presumably get pushed immediately.
If your phone is on the eligible list and not waiting for carrier approval (branded phones) use
Upgrade Advisor to get the Official release (RTM).
If you use Insider on the Release Ring, the same version is fetched as the recent public Upgrade received by these phones (10.0.10586.xxx): Phones Eligible for Windows 10 Mobile Official.
If your phone is not on the supported list, then 10586.xxx (Threshold) is the end of the line. You may as well use Insider and select Production to un-enroll your phone from Insider, but at least you have W10M. WaaS (Windows as a Service) will deliver future updates within Threshold to you without any Insider access required.
If your phone is on the eligible list and you go Insider and stay on the Release ring you get the next public version early as Release ring in effect is the RTM. This will deliver the most stable Redstone builds to you earlier than OTA push with minimal risk.
Check out this thread: Windows Insider Testing and You [Recommended Guidelines] For right now, Windows Device Recovery Tool is only flashing 8.1 to models that did not launch with 10 so it is the way to revert if you don't care for W10M: Windows Device Recovery Tool FAQ
Maybe your question needs some more clarification. What end result do you want?
Just looking in the Windows 10 Mobile Forums would answer this surface question of what build number...
