Tim and Tom, Sorry, my data on the M-K-T 1952-built cars was in error. Thanks for bringing this to my attention. I counted the stake pockets on the 1954-built cars and there are 15.
As with the other flat cars in the latest P2K production run, the M-K-T models are based on prototype photos and, as Ed says, the prototype cars were built to the AAR recommended-practice design with 15 stake pockets.
