First, welcome to OCC......
There were three generations of the 50" mower deck. There were models 50, 50A, and 50C.
Since you have a 1811, I am going to assume that you have a 50C. With that in mind, there are two options as far as the Spindle(s). You can replace just the bearing, part number
ST-745. You can also replace the entire Spindle Assembly,
959-3367 Center Spindle Assembly, and
959-3293 Outer Spindle Assembly. Those spindle assemblies come with the Drive Pulleys. You can get
759-3479P Spindle Assembly with out the pulleys.
Attached is a image of the ST-745 Spindle. The ST-745 bearing is good for all three spindles.