In RP Cyc Vol 8, there's an excellent article about 10'-0" IH postwar
cars with a detailed roster including the GN cars. With few
exceptions, GN used Morton, Gypsum and Apex RBs on all series of
these cars, so, take your pick! There's a picture of 18368 with what
appears to be an Apex RB. Author Ed Hawkins lists "Great Northern 40'
Steel Box Cars" GNRHS Ref Sheet #61 as a reference. -you may find
more specific info there.

-Tony Higgins