Do you have a serial number? If I assume it's a 346 series (because it says you're in USA) then the seat belt interlock is a parallel circuit which goes to the X10 connector under the dash. Which does indeed connect to NAMUR resistors which are built into the harness near the X10. This will plug into a speed reduction function which is programmed. This means it's not a true interlock. Also the seat belt symbol on a 346 is not supposed to be used per the operator manual so you might be chasing the rabbit down the wrong hole.

I think this model has a relay in the fuse box for the seat belt interlock.
It's not a standard open and close relay
It's a resistance type that changes to resistance values , also known as "Namur"