The hole is where the (non-head) end of the screw that holds in/ goes through the motor from the other side terminates.
So it needs a longer screw and that flush nut.
I do not think a nut is used- my example has a smaller tapped (threaded) hole inside this larger hole to receive the screw. I think this is the usual method utilized on cast AF locomotives.
Of course, anything could have happened in the past 85 (or so) years to require a nut, or perhaps a design improvement/ redesign occurred.
I'll have to check it out . I see an end a of bolt in there