Schuyler Larrabee wrote:
"Branchline offers a model of one of the cars, VERY close to being
dead-on. I don't recall the changes needed to get to exact, but they
aren't seriously challenging."
The changes involve adding side and end hatches. I wouldn't call the
models "VERY close to being dead on" as that implies accuracy right
out of the box, but you're right, the changes aren't that involved.
See Railway Prototype Cyclopedia 14 for prototype details, and read
Bill Darnaby's article in the May 1993 issue of Mainline Modeler for
the techniques involved.