Did another twist on retrieving, this time putting his dumbbell and two of his other items that look similar in shape, his big bone and his rope bone. I arranged them so that they were all in a line and oriented the same way. I sent him with a generic "go get it" and didn't point or otherwise indicate which one to get. Did this five times as I was short on time and just wanted to get it in quickly. He'd get a click and treat each time he brings back an object.
He picked out his dumbbell each time. He looked over the items a little and brought back the dumbbell when it was in the middle. When I moved it over to the right, he at first was about to pick up the big bone that had been moved to the middle, but thought again and saw his dumbbell and brought it back.
It is interesting to see this play out, and trying to see how he makes his decision on which one to get. I imagine this time he got the dumbbell because it is the object we use for only retrieving and it probably looked like a familiar task (go out, find, and bring back the dumbbell), so he settled on it.
Will do more with this soon, trying some other twists and seeing what he does.