OK. So it looks like the unit should be replaced, is what your saying, correct? And you are correct that two 50 gallons is less expensive than replacing it with one 100 gallon tank, about 17% less, at least from the one estimate I have now.
The other puzzling thing is that the AO Smith person I talked to on the telephone today said that my model number was a residential water heater, not a commercial water heater, which is how it was described by the plumber who gave me the estimate. The model number of my tank is not on the AO Smith website, as you say, but mines look identical to the drawing on the parts information sheet for models PGC 75/100 244 etc . . . The plumber who came out must have made a mistake.
Thanks for your time. This was very helpful.