Ok, the box has been opened. The set I received is the 10251 C on top. I got the M in 2006 and the S in 2008. Been a long search for the C. These were the first boxed sets produced by Marx Trains, done in 1995. The Rock Island set is commonly called the White Set. The difference is the special white hopper, only available in these sets. M is Monon, S is SOO and C is C&O. The WP boxcars came in four road numbers and were used randomly.
My experience is the M is the most common, followed by the S and the C being the rarest. Only a total of about 200 sets were made.