'42 Lithgow receiver finish, blue or park?

[Bruce_in_Oz]

I suspect that is "aniline" dye.
These are synthetic dyes dating back to the mid 19th C. Oddly enough, they were first used for colouring wood and leather.

Equally odd is the fact that they would have been discussing aniline dyes when the US had been using Manganese or Zinc as "colouring" agents for more than a decade before this.

I can imagine using an aniline dye for adding colour to anodising on aluminium, but not Parkerizing. You learn something every day.