It's not cast steel, just plain, soft, mild steel, but very well machined. As for the source, well, I suppose it's a long time ago and people/things have changed :
British Aerospace, Brough, East Yorks. ( now B.A. Systems ). Anyone with a contact at Brough could get all sorts made, just give someone on night shift a drawing with dimensions and they could make anything to great accuracy. Famous tales include titanium conrods for a Cooper S and a tungsten chess set. In the seventies, a neighbour made me various special parts for my rally car . He once said, if a Buccaneer cost £17 million, £16 million is for guvvy jobs, one for the plane !
Knowing the background of the bike and being familiar with Broughs' work, the source is pretty clear to me !
That all stopped when proper controls were put in place for access to materials, although I once went in through the security gate on an official visit, and took an item in for special machining. As I left, the security guy said " that's a first, a full bag going in and an empty bag going out ! "
