I could well believe that this chassis was constructed from scratch by a competent amateur. The sort of metal bending that home constructors were expected to be able to manage can be seen throughout, for example, Mullard's
Circuits for Audio Amplifiers which contains detailed chassis drawings for every amp in the book e.g. here
http://www.r-type.org/articles/art-003d.htm. Perhaps access to small bending machines, say at school or college, or to the sorts of small engineering companies who would do this kind of thing quickly and cheaply was also wider back then.
I haven't followed the wiring through but I would recommend making sure that the chassis is securely connected direct to mains earth. Once upon a time it was considered acceptable not to do this as long as the chassis was in a sealed wooden box and precautions were taken e.g. with the control knobs and with any wires that came out of said box. But if you plan to re-purpose it today then you wouldn't want other users to get a nasty bite in the event of a fault. We've got out of the habit of treating electonic equipment with a healthy respect.
Cheers,
GJ