Ed... I mean... with all the Duncan p'ups you've had in your hands, how in earth you can give such a WRONG info?
As most American made p'ups, the polepiece spread is 49.2mm and yes, a Throbak cover will snugly fit.
Only asian-made humbucker p'ups will have the 50mm polepiece spread.
HTH,
Just to throw a curveball in this, ...
Suhr pickups are advertised as 50mm spacing. AFAIK, they are still made in Lake Elsinore, unless something changed.