According to Jack Sim's book, Smiths were rebranded Neptunes, so you're right about the parts being interchangeable. In my (limited) experience, those parts are kind of hard to find, especially the lever that supports the handle of the nozzle.
larry --ive use before the600- 900 series nozzle holder off a bennitt--there plenty of metel there--to cut and shape-and bolt on--you only need to use the top end -you need to get creative but it not hard--
i can also use 3 complete assembly--they seems to always be broken- ruben