If you have tested loads “hundreds” of these types of sewing machines and have not come across this combination of colours before, could you possibly assume that it had been re-wired with excuse the term some odd/old cable kicking around by a DIY person?. I see you do mention it has not been re-wired but hay, just seems a little strange use of colours utilised.
Also you mention about it being an “antique Singer sewing machine” I would question if it is “Double insulated” as you mention. Back in the olden days it was possibly not even heard of
It might be worth contacting Singer or a Singer Enthusiast forum for some advice
City and Guilds 2377 Qualified PAT Testing Engineer
Always looking for new customers based in Cornwall
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