Ian, G3SEK wrote:
"No, the meter scale comes from external calibration."

Who said it didn`t?

The scale is mass produced. In the Bird element, a resistor is selected,
in some elements it is a variable resistor, called the "calibration
resistor". A suitable replacement is the Helitrim 7138 mini variable.

See "Calobration and Repair for Bird Wattmeter Elements, page 48, April
1989, 73 Amateur Radio.

Best regards, Richard Harrison, KB5WZI