Thank you all for your responses. The model number is 991080, serial number is 011261. The belt looks good. It is just loose. I can take pictures if you want. The mower has 371.0 hours on it. Thank you for your help.
I have a Gravely Pro-turn 152 and the belt spins two of the blades is very loose. What tightens the belt? I just don't readily see an easy way to tighten it. Or, do I just need a new one? It has 369 hours on it.