I would go with a IH481099-R2 per this pdf chart which is hosted here:
www.cubcadet.info/Manuels/Belt-blade_&_PTO_ref_guide_1947-2004.pdf
page B5
Being the 125 was not built after Aug. 1969
I do not know why but the IH-489399-R1 is for a 48" narrow frame tractor and it has the loop style skis instead of gage wheels.
Built after the CC125 ended production.
The earlier narrow frame 48" deck does not have the skis and has gage wheels that are on brackets welded onto the deck outboard of the tractor tires. and uses belt IH-481099-R2