From information that I have gathered, and going from memory because I am at work.....
The small, 36' Monad was used starting with the arrival of the first all steel box cars in 1940.
The "Mainstreet" slogan was first used on new box cars under the 36' herald in late 47 or early 48, let me verify.
Some wood box cars would get a 48' Monad and "Mainstreet" slogan starting around 51. I have seen a poor picture of a wooden truss rod box car with the 48' Monad and NO slogan, but only one.
I have NEVER seen a wooden car with the 36' Monad and "Mainstreet' slogan. Also, I have NEVER seen a steel car with the 48' Monad and slogan. If someone has pictures of either, please pass them on to me because I would be interested in them. I'm sure that it could have been done, there is always one exeption.
In 1951 new box cars got a 60' Monad, with the "Mainstreet" slogan.
I have seen one picture of a 1937 DS box car with a 60' Monad.
The Railway was added to the Monad around 1955, however this is after my period of interest, so I do not care.