Okay. That may explain it.
Sheaffer did make a pen in that body style with a 23K plated nib BUT it should have an opal on the top. The nib and section either came from one of the few models that used that nib or the opal came off your pen.
From Sheaffer Targa dot Com
The regular fluted gold plate Targa should have a 14K nib.
Jar , the link you provided says the Targa with opal fitted into end cap also came with 14k solid gold nib. The plot , thus , thickens , instead of being resolved.lol.