Both versions have screw on cap pens with the section and filler almost totally hidden in the body. Since you hold the body of the pen to write even the standard size feels large.
They both use the same system to hold the section in place, a lock at the base of the body that can be opened and used to unscrew the section. This allows the section to be removed cleanly and easily.
The large sized Montparnasse is really a handful. It is a commanding pen that will stand out in any gathering, yet one that also is clean, simple and without any ostentation.
As with all ST Duponts, the nibs are moderately wet and very smooth, very controllable.
The Montparnasse was the last of the ST Dupont pens to use a cartridge and converter other than the International size. The ST Dupont converter though is the same size as the Parker and Aurora twist converter and they are a perfect fit. Since it is nearly impossible to get the old style ST Dupont ones it's good to know that the Parker will fit the Montparnasse, Classique, Gatsby and Lady models.