This is a Treadmill Doctor Replacement Drive Belt Part Number 130-0121 for Various Startrac Models Listed Below.

Star Trac- 
9-4032-BUSAP0
E-Series - 9-9151-MUSAPO
S Series S-TRc - 9-3581-MUSAP0 G2 - 110V Ver. A (TRSC)
S Series S-TRc - 9-3581-MUSAP0 G2 - 110V Ver. C (TRSC)
S-TRx - 9-3573-MUSAP3 (TRSXxxxx-Lxxxxx)
TR3900 Series
TR4000 Pre-Soft
TR4000 PreSoftmill - 9-4011-BUSAP0 - Grey (4xxxxx)
TR4000 PreSoftmill - 9-4031-GUSAP0 - Grey (4xxxxx)
TR4000 Softmill 
TR4200 - 9-4221-SJPNP0
TR4500 Series