Looks like a Parker 51... they were made for over 30 years ... wash it out with warm water and then fill it with ink and see how it writes. If 18K gold it's worth at least the weight of gold ... if say 15g = £365 ish ... You could ask about it on a Parker pen forum ... nib is a normal hooded type ... it's a regular nib once dismantled but dismantling is not recommended.
http://www.fountainpennetwork.com/fo...-parker-forum/
dunk