Actually you answered your own question. Yes the round collar came off the plate and a regular puller will remove it from the taper on the crankshaft. You will need a new collar when you reassemble.

"Could it be that it had a round collar welded to it and mine came apart and stayed on the shaft?"

I believe that tis is exactly what happened. The "round collar" you are seeing should be attached to part #56. With the collar attached part # 56 is a press fit on the shaft and is removed by hitting it on the edge with a hammer to loosen it. You're going to need get a new part 56 and probably cut the "collar" off with a chisel or Dremel.

Yep. Sure enough, there it is:
http://cgi.ebay.com/LAWN-BOY-LAWN-MOWER-DECK-BLADE-ADAPTER-677525-/260277265986?cmd=ViewItem&pt=LH_DefaultDomain_0amp;hash=item3c99bbe642

I guess, as a rule of thumb, one should just always research Ebay before posting here!

This could be an ultimate test for good ol' P.B.Blaster Super-Duper Penerating Catalyst

..otherwise, suppose a torch, ViceGrips & a hammer might do the trick?

Here is a link that might be useful: Part Photo on Ebay

Put the PB Blaster on and let it soak overnight. Have everything ready the next day and heat the thing quickly, then break it loose. You want to take it easy on the engine bearings and try not to beat too hard on the shaft. I'm thinking if you slide a piece of pipe the right size over the crank and tap it, it'll move toward the engine. Once broken loose, a puller should easily take it off the shaft. If you can rig a slide hammer that would break it loose and remove it at the same time.

I took it apart again, got a mirror and light underneath and it's no surprise why none fit. It looks as though it once may have been a hex head but, basically round, it barely resembles one now.

I thought I may be able to turn it with an EZ Out without much luck.

I already attempted to access it from below by removing the motor from the deck but a broken MOWER DECK BLADE ADAPTER put a snag in that plan.

I have the exploded diagram but can't really tell based on that alone, will it be possible to, leaving the motor attached to the deck, remove sufficient parts in the surrounding area to be able to get pliers or visegrips around the outside of that setscrew?

Okay, either you post some pictures or forget it, because I am totally lost after these apparently conflicting halfast descriptions, including a link to this very same thread. That has to be a first.

I'm confused now too. It just sounds like the square blade adapter plate has broken loose from it's collar and left the collar stuck to the crankshaft. This is not unusual. Soak it as described above and pull it off with a geat puller. Maybe add some heat. Use anti-seize when you put the new one on and you'll never have this problem again.

Because these two issues, though related, are technically two different issues, I created two separate posts. I then accidentally responded in the wrong thread and this forum, unfortunately hasn't the option to edit/delete one's post. (By the way, no conflict or contradiction and, I think, my descriptions were well written and far from (I assume you intended) "half-assed")) Sorry for any confusion.

This problem has been solved.
The Mower deck blade adapter came apart. If I want to get past it I need to, one way or another, remove the stuck collar from the shaft and replace it. Case closed.

Here is a link that might be useful: Dealing with the Stripped out setscrew is another story