Flexible nibs were common in early fountain pens, but were already becoming less common in the 1940s.  

The flexible nature of the nib allows you to vary the width and "wetness" of the writing line by varying your pressure on the nib.  This can produce very interesting and attractive handwriting, but you have to acquire the skill to use a flexible nib effectively.  

A flexible nib is not for someone new to fountain pens unless you are determined to learn to use it.