I'm not very sure though that it's assumed that you registered at copyright.gov instead of WGA. Because the format as you tell me is the same!
I'm talking professionals here. If you're aiming to get paid, occasionally you need to sue. registration with the WGA doesn't help in this regard. Copyright.org can. This is why it's assumed that you've registered your material with copyright.org and you're not an idiot.
Yes the format IS THE SAME. The format is the same even if you fail to register your script with both.
The way I suggest is not putting ANYTHING to say the script is subject to copyright protection, regardless whether you register it with copyright.org or the WGA. The format is to put nothing either way.
In fact, you don't need to register it with copyright.org and/or WGA and can still put the whole "Copyright 2015 Random Joe". It's partially for that very reason that I'm saying to leave it off. Stating Copyright on written material has not been necessary for many decades.
While I cannot attest to this, some say that if you include the WGA registration number on your script, it's a big sign that screams "run away, amateur writer".
mlesemann is right though, you're overthinking things. If someone submits a script to me, it's not something a subject worthy of my attention. A script with copyright or without won't matter either way.