B-4/B-4a Box Car Question

[nw-mailing-list at nwhs.org]

Greetings,

   In doing some research on B-4 box cars, I have come across 2 different 
roofs used on these cars. I am curious if during production, the ribbed roof 
replaced the smooth peaked roof on later series, or if this change occured 
later in their service careers. My main focus on modeling is around 1950 or 
the early 50's, and also wondering if it would be safe to model certain 
number groups one way or the other.

   Also, is there any noticable differences between the B-4 and B-4a box 
cars on the outside?

Thank you and have a good day!

Michael Stovall