It turns out that I misread the engraving on the nib. It is a product of W.J. May and here’s a better one:
If I requote Rard’s comment that will give you an idea of how they fit in the British pen business and how rare they are today.
It is not a Mau, it is a May’s. May’s were a brand made by the Wade family (were involved with Summit, owned Lang’s, Pronto, Regent, Securus etc). May’s fountain pens were simple, basic lever fillers and there are not many of them out there. I think I may have one of the largest collections in existence owning only five.
They made some decent pens and were very innovative, producing the first capless fountain pen, the Pento Capless – very rare indeed. I’ve never seen one.
Many thanks to Rard Changizi for the photos and information.