I have the answer.
Directly from Kubota Corp:
"Thank you for your email to Kubota. The GR2120-2-54 & GR2120B-2-54 replaces the GR2120-54 & GR2120B54 models, with adding a muffler shield. The GR20 Series has been upgraded to include rod steering, a hydraulic PTO clutch, and improved transmission, providing greater durability.
Sincerely,
***** my edit, kranky
Customer Satisfaction"
Cheers!