Yep, it's pretty much RTM. But it's not the build, which is going to be delivered at July 29th. Msft will add some patches to it & deliver it with different delta (the number at the last of build string) version. In simple words: Is Build 10.0.10240 RTM? - Yes. Is Build 10.0.10240.16384 RTM? - NO. Msft may add some bug patches & might change "16384" delta to something higher.
other question, which version do I need to install in order to keep my activation? I have no idea because my windows 8.1 just says that, It doesnt say if it is Single Language, Home, Pro, nothing, just "Microsoft Windows 8.1". I've tried SystemInfo, WinVer, etc. Edit: I've tried to install Win 10 build 10240 Pro and it asks for CD-KEY
Build 10.0.10240.16384 might be the RTM many sources said its RTM but not Microsoft... this version is probably "RTM" for Insiders it might be the same version as real RTM I duno but because there tracking / telemetry services, it might possible its not RTM for normal users, ms keep quiet about this so there doubt