Rain,
Are you saying that some central bobbin machines had a rotary mechanism? I've inspected both of my 15Ks and the hook definitely oscillates.
Theory number 2 about why American machines had the Indian Star... because Singer in Scotland were unable to export to India, they shipped the decal sets to the USA where they were put on machines for the American market and sold as a special order. In other words, the decal was intended for the Indian market, but the machine it ended up on never was.