Well a big thank you to all for helping with my question. I went out to the workshop but found that I had no silver steel. As it's Sunday I decided to have a go at turning it straight down from a piece of 6mm brass as suggested. I shocked myself with a first time success! It seals better than the original item. I was mindful of the information regarding the lack of strength of the core metal but it seems OK, also bearing in mind that it never really has any load on it.
I don't know how they made them but I bet the process never modernised from the original. So maybe they were always turned
Thanks again all. That's another piece of history back on the road. It's a Moto Guzzi Superalce by the way. Built for the Italian army from 1946 to 1957.