The Columbus model 504 was manufactured for Columbus Radio Centre Ltd by Radio Corporation of New Zealand Ltd in 1955.

5-valve mantle radio

1955 Columbus model 504

1955 Bridisco Crystal SetInterestingly, around the same year an English company (The British Distribution Company, or Bridisco) released a crystal set with a remarkably similar design, seen here 

Valves (5): 6BE6, 6BA6, 6AV6, 6AQ5, 6X4

Intermediate Frequency: 455kc/s most likely

Frequency Bands: 1

Chassis Notes(most schematics can be clicked to download a full size version)

1955 Columbus model 504 Schematic

General Construction Notes for Radio Corporation of New Zealand Ltd: The first digit of the serial number typically indicates the year of manufacture of RCNZ chassis' (although not the decade - that requires a little knowledge of the valves, construction, etc). Sets from around 1934 onwards were often (but not always) constructed in a distinctive pressed 'baking pan' style chassis, seemingly unique to RCNZ.