Bungy,
The following is a copy of a post that I wrote over on the other Chevrolet forum a few weeks ago when a similiar guestion came up. Also, the owner of one of those cars on ebay, the original 6 cly. posted a revised description stating that it was a "built" car. I hope this answers your questions concerning 1961 Impala SS's.
Every now and then this question comes up. I don't know if Chevrolet intended to create a "mystery car" when they built the 61 SS but they sure did a whale of a job!! I've made this statement before but these are probably the hardest cars to correctly identify and the easiest ones to clone ever built, all this while being some of rarest and most desirable cars produced by Chevrolet.
Let me attempt to answer your question. RPO-240, Impala Super Sport Equipment, was available on any 1800 (Impala)series model except the Nomad station wagon. The equipment consisted of SS emblems on the deck lid and each quarter panel. Solid center full wheel covers with 3 bar spinners, the wheels were 14"x5" and painted black on all SS cars.
The interior equipment included a grab bar on the passenger side and a trim plate or console when the 4 speed transmission was ordered.
The following options were MANDATORY before the SS option could be ordered from the factory. Heavy duty shock absorbers on the front & rear, heavy duty springs, front & rear, either a heavy duty powerglide transmission or a 4 speed transmission if you chose the 305 HP motor. All other motors required a 4 speed transmission. Also required were: power steering, tachometer, dash pad, power brakes, metallic brakes, 8.00x14 1" wide whitewalls, and one of the following motors: 305 hp, 340 hp or 350 hp 348 cubic inch or the 360 hp 409 motor. According to records, there were 453 of these cars factory built by Chevrolet. There were 142 409 cars built in 1961. To my knowledge, there are no records to verify how many of these motors were installed in the SS cars. It is my opinion that some 409's were installed in SS cars but I would guess that most were installed in the cheaper, lighter body cars because they were designed to be raced. The drag race guys mostly used the lighter bodies and the NASCAR boys used the Bel-Air bubble top bodies.
I hope this answers most of your questions. This information is available from Chevrolet and was originally sent to dealers in a memorandum dated Feb 1961. Since there were only 453 factory SS's originally built, and there were approx. 6000 Chevrolet dealers in the US, think about the difficulties that one of those dealers faced in getting one of these cars!! Contrary to popular belief, there was NO dealer installed SS option, however many dealers installed the emblems, grab bars etc so that they could sell cars. If you find a 61 Chevrolet Impala that was built before Feb 1961, doesn't have the above options and it has a motor smaller than a 305-348, you can bet that it isn't a factory built '61 Super Sport.