The Parker 180 and the Parker Classic Fountain Pen are often confused with each other due t their many similarities. Both are very slim with a long slim nib. The Parker 180 measures in at 132mm, the Parker Classic at 135mm with a slightly longer clip screw on the pen top.
The main visible difference is the nib.
The Parker 180 has a long thin double sided nib, either F/B (Fine on one side and Broad on the other) or X/M (Extra Fine and Medium) marked on the bottom of the nib. The nib is unshrouded with the sides visible all the way back to the chrome collar of the shell grip section.
The Parker Classic has a semi-shrouded single sized nib. The black plastic nib shroud that covers half the length of the nib is the main identifying characteristic of the Parker Classic, with the nib size marked on the bottom of the shroud which separates the nib from the chrome collar of the shell grip section