Is This A Parker 51?

[a-pen-for-a-sole]

I was gifted this pen today by my grandmother, which I assume hasn't been used in quite some time. I think, based off of pictures, that this is a Parker 51 vacumatic. I'm not sure how to get the nib out, or open it up. The top part of the barrel either doesn't or won't twist off, so I'm hesitant to try anything since I don't know what I have here.

 

Thanks in advance for your help!

its a prarker 51 vaccumatic. its a great pen. i think you have to restore the pen. send it to some professional, and do the restoration. now you are owning a parker 51 and entering into a parkers family. welcome to parkers family. . i also became a family member of parker very recently by buying a parker super 21. now a i am planning to buy one parker 61. one last word, " you have to be immensely thankful for your grandmother for giving you such a great gift". my regards to her. enjoy the pen.