To Quote the advertising literature of the Time "CTS speakers feature 1. Patented Aluminex laminated voice coil form for maximum power dissipation. 2. Thermosetting adhesive throughout the voice coil and cone neck joint for long life under high power use. 3. Barrium ferrite magnets for maximum efficiency — minimum profile. 4. All speakers designed with 8 ohm nominal voice coil impedance measured at 400 Hz. 5. Quick connect-disconnect spring pressure terminal posts* solderless connections. No special amplifier lead wire or formings required. 6. Hammertone-Grey, high lustre baked enamel finish on housing and magnet structure. 7. Individually packaged to ensure maximum protection in shipment. 8. Red terminal post on each speaker denotes positive phased terminal.9. All CTS lead guitar and organ speakers feature vented aluminium dome dust caps to relieve air pressure generated in the area of the gap,
15E28L lead model and 15E28B bass model. Big sound from 15" diameter cones, rugged construction features 28 oz magnet 1 %" voice coil and handles 60 Watt R.M.S."
Thanks to Archive.org & Electronics Australia magazine 1974
https://archive.org/stream/EA1974/EA%201974-03%20March_djvu.txt