In tip 2047, we learned how to manually check whether Service Pack 6a has been installed in Windows NT 4.0.
The standard method of inspecting the CSDVersion value name at
HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows NT/CurrentVersion does not show the a, showing only Service Pack 6.
If Service Pack 6a has been installed, the Installed value name at
HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows NT\CurrentVersion\Hotfix\Q246009 will contain a 1.