In 1955, GN changed the slope sheet angle on some of their 73000-73199 hopper cars:http://www.accurail.com/accurail/art/2300/2301.jpg
from about 30 degrees to 55 degrees, and renumbered them into the 73700-73764 series. The cubic capacity dropped from 1622 to 1253.
I've asked after this matter on the GN group. Nobody knows why.
So I'll bring the subject up here.
What load would GN have modified these cars for?
I'm interested in factual knowledge, and also informed speculation, since I have doubts anyone knows specifics on this matter.
My suspicion is that it was for "ore", or what most civilians would call "dirt".
I will also speculate that if the "ore" were no longer being transported, they'd be prized as hopper cars that would dump just about anything.