An "un-sized" portion of the rear of the neck is not necessarily a doughnut.
If you use the technique I suggested in an earlier post, resizing the case in an expander-ball die, the doughnut will show up on the outside of the base of the neck, and be measurably larger than the rest of the neck. Setting the neck-turner to just skim most of the neck will eliminate the doughnut if you just turn the very top-end of the shoulder.
