My understanding about the blackside stuff is that there were what they call running changes. They would just dump new parts and accessories in the box and you might just have something that the person who put it together just grabbed different vintage parts as they assembled it originally.
Or I suppose it's possible some were replacement parts. I think they went to the blackside during the war years when chrome was needed for war material not the home front. Tho I'm not sure how many domestic sewing machines were produced. Somewhere I saw a badged RAF (UK Royal Air Force ) machine, don't remember if it was here :)