Okay here are my thoughts on this pen...
Well the nib is from a De La Rue Onoto obviously. I don't believe the pen is. That nib may be 14k gold. You should pull it out and check, be very careful not to bend or damage the pointed end of it. The nib is probably the part part of this pen. The pen itself "resembles" a Waterman's No.52 or maybe even a No. 5 in styling but ... If there are no other marks other than patent dates and the generic clasp manufacturers names the it may be a generic run of the mill pen. There should be some identifying marks on the pen itself, unless it has been damaged and you cannot read it. Any quality pen/pencil maker would have their mark on the clasp, band, nib, and sometimes the body of the pen itself.They would not use Argus or Ideal parts.
Oh yeah, it's "hard rubber" just as you said.