I don’t remember having seen a Mayfair before. A few possibilities: it looks like a European copy of the Waterman 52.

The nib is an 18 carat warranted, so possibly a British-made nib for the French market. Nibs can be replaced of course, so not always a guide to the origin of the pen.

Langs named several of their pens after London locations. Did Langs ever use a box lever? (This would be a much better blog if my memory worked better.)

With the box lever and the broad cap ring it is clearly a high quality pen. I’m told that the nib is a very pleasant broad stub.

The Roman Numeral IV is on the cap top. The lever has what appears to be a castle on a hill (?), with the word WIZ over it , and a crescent moon.

Thanks to Rob Parsons for photos and information.