That kit adds alot of detail.
I know for sure that the 251/1 was used as an ammo carrier, only it is not a 251/1. A 251/1 is a troop carier, the ammo carrier is a 251/4.
here is a pic of one
http://www.geocities.com/motorcity/pit/3515/251/251-4.htm
this website http://www.geocities.com/motorcity/pit/3515/251/#index
says that the seats were probably removed for more storage room.
this page has areview of abook you might want to try with 2 interior pics.
http://pmms.webace.com.au/reviews/books/groundpower/gp251.htm
here is one more extirior pic
sorry I couldnt be of more help.