I am surprised that the Parker 51 is the correct pen for the holder. I have the round paperweight one, in a browny/green colour (definitely not cinnamon) with the original pen, definitely the original pen as it is a family piece. The pen, ball point, is black to match the holder and has a gold band around the middle and a long tapered gold "tip" on the end (the none writing end) with a little bobble. It is a Parker but it's most definitely not the 51 or even a version of it.
As a total aside, when I was little I used to love the "pull the pen off the holder" game... I am glad I didn't damage it!