Ok, so finally I can see what is going on. A previous owner has switched out the standard brass fittings at the ends of the chrome bar for simple end caps, and these caps do not have the brass flange holding the bonnet halves in place. A quick run up the road to Morgan West showed exactly what was going on, helped make sense of everyone's descriptions, and now two shiny new brass end brackets are on order.
Thanks everyone.
dave
