Well I think I found a parts diagram for this mower frame, on a UK mower parts website:
Victa Pro 460 - 18" utility side discharge up to 9/1999 The blade disc is different, and the wheels may not be exactly the same (mine has 150mm wheels front and 175mm rear, all with 6902-2RS sealed bearings). But the CH85124K baseplate seems to be the same and apparently all the other fittings are the same.
I stripped mine down today, finding that the bolts that hold the handle and axle assemblies to the baseplate had corroded from all the wet grass. This made removal of some of the nylock nuts difficult, or in a couple of cases impossible. Maybe I'll replace with stainless steel versions.
Under the veneer of old grass pulp caked on the underside of the baseplate is the beginnings of significant rust. I've wire-brushed a lot of this off, but there are areas of deeper rust I still have to wirebrush out. Then I'll paint with Killrust epoxy. But there seems no realistic way to stop the paint coming back off in the first few minutes of mowing and have the rust start up all over again. Unless there's some dodge anyone knows (Fish oil? Some brand of proprietary rust-converter)?