I really admire the work of Ronald Searle, and would like to find a pen as capable of fluidity and varying thickness as what he has mostly been using for years.

Today I discovered he uses a Mont Blanc fountain pen with a good amount of flex in the nib, which can draw a thicker, rougher line when using the back of the nib. Most of the pens I know draw thinner lines upside-down, and I'm new to vintage FP's so I thought I'd ask the experts if this sounds familiar at all. It would almost certainly be a model from pre-60's.

You can see some of Searle's work here

Any ideas which Mont Blanc's I might look at?