I don't think that anyone is questioning what the specification stated - but we can all quote numerous examples of the specification / drawings being honoured in the breach.
It would not be unreasonable to suggest that the majority of contributors to this thread are ABSOLUTELY convinced, by the photos posted here, that the battery boxes were painted green, either by North British in 1958, or very shortly after delivery.
That being the case, the sensible course would be to supply the models with green battery boxes; those who remain unconvinced can easily apply a coat of Halfords satin or matt primer.
The other alternative, of supplying them painted black, would require those - (the majority?) - who require green battery boxes would have to try to match the shade of green. Not an easy task!
Regards,
John Isherwood.