JSI Tip 2801. How can I tell if SP6a is installed, using the registry?

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.