The thread about the '73 Nova SS got me thinking about two '61 Chevies I owned years ago.
I had gotten interested in the '61-'65 409 cars, since I had an original '63 425 horsepower car. In the process of buying and selling early Corvettes and Impala project cars, I happened across an early '61 SS. I found out these are very rare, and this reminded me that a friend of mine in Illinois had a red 3X2 348 SS years before, so I bought it.
A few months later I found a second car, and bought it. I don't know where they ended up, as I sold both unrestored.
I found the research interesting. There were only about 450 factory built cars, all equipped with the 348 or 409 engine. The majority of the SS models were Implalas that were "dealer equipped" with a package the factory sold them. The factory-built cars can be identified by a single or double "S" stamped on the cowl tag.
The difference between the Impala and SS models was generally a 348-409 grill emblem, modified spinner-style hub caps, a grab bar on the dash, a column mounted Sun tach, and SS emblems on the quarter panels and trunk.
Here are some pictures I found on the net.