You can search for them in
HKEY_CURRENT_USER\Software\Classes
according to WinSuperSite, you can also find their contracts in HKEY_CURRENT_USER\Software\Classes\Extensions\ContractId
which contains things like Background Tasks, File, File Picker, Launch, Protocol, Search, and Share.
For more internals of the Windows Runtime, see this video by Microsoft.
The reason why they don't make this easily available is most likely because they try to mimic the Google Chrome approach where version numbers no longer are that relevant, see also: Jeff's post