I am assuming this is a Conway Stewart fountain pen because it has a nib which is marked "Conway Stewart, 14Ct Gold" and I found it in a Conway Stewart box but I have failed to find it in the Book of Numbers. The pen is finished in a black and green marble with gold plated clip and 3 cap rings. Straight lever and sac will need replacing. Nib is medium in size. pen is in a good shiny condition but has a 6mm gouge near top of barrel as can be seen in photo. Box is rather battered but a Conway Stewart  instruction and guarantee leaflet is in the bottom.
If anyone can shed any further light on the model number or even if it is a Conway Stewart, please let me know and I will add the information to the listing.