According to the info in
http://www.lorentz.leidenuniv.nl/history/e...n/einstein.html, in 1921 Einstein gave Ehrenfest an unusual present: The fountain pen he had used to write down his research on general relativity. It is now on display in the Boerhaave Museum in Leiden. The link in that page is broken but the image is the one below.<p>(It looks like a flex one )



Then Paul posted this image



and JRG offered these two links



I thought it should be kept somewhere for reference.

Thanks, Antonios! It's nice to have things archived so that others can read it later, isn't it?
Anyone know what type of Waterman pen it is?

Yes this is why I enjoy FPN. Sharing the knowledge is one of the most enjoyable aspects of this hobby.
I think the pen is a Waterman 22 taper cap, c. 1900-15 according to Vintagepens.com (link) it looks like the middle of the three shown below.


Einstein pen shown in the Dutch museum probably has an add-on clip.

Indeed... sharing information is the goal of the community. Thank you Antonios.