Stephen,
I received my cap yesterday, but it had great trouble fitting the Master Cylinder that I had taken out of this car a few years ago.
I called our friends in East Sussex and they suggested that I use a rotary brass brush (used my dremel) to remove the coating off the threads.
This did not work and the cap only goes on about half its thread depth.
I have an awful feeling that the thread is slightly tapered (see pics)

- CM250430-095751003.jpg (225.07 KiB) Viewed 1556 times
but using a caliper and a jewellers eyepiece, I cannot put my hand on heart and say that the original cap has a taper.
I will uncover my project car, today, and access its Master Cylinder to see if it fits in there, as the 1950 MC (that I removed) may well have thread problems beyond the depth that the normal cap goes down to.
I had tried the old trick of working the cap backwards and forward, but it then just jams after a certain point, as it would if the thread were tapered
By the way, the cap I received looks a lot better than the image on the website, is coated black and is made of aluminium.

- Cap comparison.jpg (450.91 KiB) Viewed 1556 times
George.