As best as I can interpret Jon D. is the winner with the correct guess. The extra bellows are there to represent the those of an earlier incarnation before a rebuild.
To quote the pferd's reply:
"Hello Stacy,
thanks for your question.
I can not find the item no. of the wagon. In the real railway the last car has a wider bellow. Therefore, we add these to each car for a correct repositioning.
In the manual on page 4 you can see the slim bellow and on page 5 you see the wide bellow.
Sincerely yours,
your customer service employee
Selma Reisert"
This response prompted me to actually open up a bag with the extra bellows for a more careful examination. Attached is a shot comparing the two. The extras are quite a bit shorter than those that come on the wagon. The width is actually the same. Depth of the extras is the same at the top. They are a skosh deeper where the footplate is molded on but still not as deep as at the top.
I'd like to thank Stacy Cousins, US (North American?) Märklin Customer Service Rep for her efforts and very prompt replies!