Tim,
That looks just like the cover I have for my 56 BN-2. Mine measures 29 inches long (at the top). If you look at the Moss catalog I think you will see an error. They reference the short cover for the BN-1 as item #1, but then when you look at the list it has no item number and they assign number 1 to the BN-2. I'm going guess that all of the side shift cars except for the BN-1 had the same cover. The one you have and that I have is for the BN-2(of course this would not be the first time I have been wrong about something like this....) Are there more than 3 styles of cover?