Well you learn so much on these forums. Immediately after I posted that about the round butt, I looked at the other 2-1/2" 66's posted and thought to myself, gees some of those others pictured here might be round butts too. The various custom grips kind of concealed what was under them to the untrained eye, mine included.
All that I do know is that I like model 66's, have always liked the model 66, have owned a few, and will never be without one again! Back before the internet, or at least before I had internet, information was very localized and restricted. Had I known that the smaller K-frame Magna style grips would have fit a model 66, I would have not parted ways with my first model 66's due to grip size. I might switch to rubber Pachmyers but never switched to a smaller grip. My current 66-1 is the way I like it. Most of my early model 66's were 6 inch models. They look great and shoot very well, but are slow to remove from a holster (not talking about QuickDraw stuff here) and a bit bulkier to carry. I really don't think that I would make a 2-1/2" model 66 my everyday carry piece or my back up hunting gun. I have a 5 screw M&P snubbie and my 4 inch 66-1 for those uses respectively.