What you have are examples of the 1959 FOx Rocket .35, a derivative of the '57-'58 Combat Special. It has the timing of the CS sleeve, but a standard (unhardened) crankshaft, so while power output is upped, durability isn't. It should be regarded as a good "sport" engine, but kind of a mongrel; neither the stunt run of the .35Stunt nor the long-term performance of the Combat Special. I believe Fox created this engine to compete with the McCoy .35 RH and K&B Stallions of the period.
My suggestion, if you want good stunt engines, is to eBAY these (especially if you have boxes and paperwork) and buy the good stunt engines, whether FOx or something else.