The Z-section horizontal ribbed hopper cars were built by Standard Steel. The pressed-steel, hat-shaped horizontal ribbed hopper cars were built by the Pressed Steel Car Company.
The Susquehanna purchased 1,125 of these cars beginning in 1912 from the Standard Steel Car Company. The Erie purchased a number of these cars beginning in 1914 from the Standard Steel Car Company beginning with car number 31000. The Erie then bought another series of these cars from the Pressed Steel Car Company beginning in May 1916, beginning with car number 32800.
In 1925, there were 2,483 of these cars in service on the Erie. I think it's safe to say that car numbers 31000 through 32799 had the Z-section horizontal ribs, and 32800 through 33499 had the pressed-steel, hat-shaped ribs.
If you look on pages 270 and 271 of my book, you'll see builder's photos and in-service photos of these cars.