Plus, this seller has a TON of pens for sale which leads more views to thier auctions. I just bought a Watermans parts pen just for the nib for $10 yesterday. (Missing a clip, but all there for the most part. Nice spare barrel if I ever need it!
Very odd pen indeed, I wonder if the pen really had a silver jewel or if the pen had a broken off black one. The pen does not have a stepped body and the section is black which would indicate that the proper cap would have a black jewel?? However, as said... the seller even noted that it said Esterbrook on top so that would not indicate a broken one. Either a transition between the two styles or maybe a franken pen? I dobut the later though given the body.
PS, this pen is NOT visulated, below is a photo of one that is. Very easy to tell on these, it's not like the Parker pens that have it in the body itself.

Photo from Esterbrook.net