Bill, you are so right. I spotted an aerobatic Bonanza in Trade a Plane a while back and assumed it was a 35. I just checked Bud Davidson (Air Bum)and he says that 25 aerobatic E33-C were built and sold around 1970.
But back to slips. I saw an airshow aerobatic routine flown by an Ercoupe and they are rigged to not be able to slip. He crossed controls several times.
Bob