I cant say this for certain but i have no memory of ever seeing an isolated variac, that is, a variac winding over the top of the primary coil, which is a shame because traditional variac transformers are rather inefficient by weight, due to the difficulty of getting a high copper density. It would be rather easy to fit a primary coil under the secondary variac, if the inner diameter of the core were increased a bit so that the secondary coil is a standard 2:1 inner to outer ratio suitable for machine winding a variac
I had a burned up 0-260vac 10 amp rated variac, i rewound it with 15 gauge wire and i think i can get 140vac out of it at 4 watts of no load losses.
I think the original 260vac winding was 20 gauge wire, but it might have been 18.