The seller has proven reliable to me in the past, but as you say, it's a lot to consider without more detailed photos/information. I suppose the answer would lie in dissecting the process of pressing vinyl, step by step, and see if the error is feasible. As far as 'promo' goes, perhaps it was discarded from an initial production run and just slapped with a white label. Intriguing, nonetheless, when you consider this error on the Sandie HiG pretty rare, combined with it potentially being some sort of promo for Thorn. Megarare!