It's the long belt (an A74) and runs from the motor pulley to the mower deck out front where another belt then connects power to the blades.
As that long belt passes under the operator's seat it goes either under or on top of a horizontal bar. What I'm asking is if the belt goes on top or underneath that bar.
Anybody with a rider 850-12 could take a look as the parts diagrams are not helpful on this subject.
JohnK
January 2013