Fox first made this engine for Comet in 1959. Around 1961, Fox and Comet got into a battle and Fox took all parts out of Comet. Having a huge stack of parts, Duke cut the venturi off at an angle, reshaped the little die ejector tab under the bottom of the venturi, cutaway the tank body to leave a rather crude rear cover, ground the Comet cartouche off the case, and Voila, your Fox .07 Compact Stunt Motor. Peter Chinn reviewed this engine in the July 1961 Model Airplane News. Doug Martin Foxengines@aol.com can supply you with glow plugs and most parts; no pistons and cylinders. The .07 was sold without a tank.
In 1962 Fox destroked the .07 and made his first .049, it looks just like the .07, but has a little rivet head on top of the piston and 7 instead of 8 cylinder fins (review, P. Chinn, MAN 6/1962). The .049s were sold with and without a tank.
In 1964 Duke redesigned the .07, made a nice shiny tumbled case, and came out with the .07 RC (P.Chinn, MAN, 5/1964). This was closely followed by a new .049 with the shiny case (P. Chinn, MAN, 8/1964).
Finally, although advertised in 1963, in 1964 Duke introduced his .049 FAI SPECIAL, an entirely new .049 with a larger diameter crank, different intake and exhaust porting, and a new case.
Also, Fox didn't supply these after OK went out of business. The Comet RTFs were just too heavy for the Cub .049s. So Fox supplied an .07. After the Comet Fox battle, Herkimer (OK) stroked their .049 to make an .060 and that is what went onto the last Comet RTFs.