The main identification marks are the slanted plug, off-center (wedge-head combustion chamber) and 4-bolt backplate.
They came in two displacements, .29 and .36 On one size,displacement was stamped on an engine lug. If it wasn't there, you had the other size, I forget which was which... They were an at least ounce havier than the Stunt 35. They were built to be more sturdy - obviously intended for greater power. Fox's recommended higher nitro fuel suits that.
I found them considerable 'shakers' when new, but Marvin Denny reported they smoothed out a lot with more run time on them.
A stronger sport engine, not up to a K&B or Supertigre, but less expensive.