Mannyrock...you almost have the name right...its "Plug Door" boxcars referring to the type of door and (I always thought) the fact that on the model the doors don't open but that is just my interpretation of the name.
The General Mills boxcar ... can you find or remember its number...most of the price guides are organized by product number. The number I am referring to is like the "9339" number on the side of your Great Northern boxcar. If you have that number I can give you the price guide value right away. If you don't, I will look at my boxcars and see if I can find one that matches the General Mills name and then get the number that way.
Don
