Thank you all. I really do not think the cat was in serious condition for a long time. He had three other "families" that watched out for him. I think any of us would have noticed if he had shown signs earlier. I keep telling myself that at least.
He was a happy cat. You could tell that. His real family did provide him with the basics--food, shelter, basic medical care (he was neutered, tattooed, brushed, no fleas). He just sought affection from others. I think he knew he was loved. This isn't saying that he wouldn't have been a happy inside cat if given that chance from his kitten days.
Any lessons learned? No, not a one. I will still care too much. I will still butt my nose in where it maybe shouldn't be.
And Jim, I do know what you mean. I don't think there will be any changes in behavior.
Know that where I live this is common treatment for outdoor cats--and there are many. At least I know I did make a small difference in one special kitty's life.
I thank you all. I know my little friend is at peace. And I know that you all understand.