The O.S. twin-stack spark ignition 29 is perhaps the most overlooked O.S. model of them all - even O.S. themselves appear to have forgotten it, since they omitted this model from the O.S. engines manufacturing timeline pages on their own web-site! The engine was clearly a direct ancestor of the familiar O.S. twin-stack glow-plug models which begain to appear in 1949. 

The O.S. twin-stack spark ignition 29 represented such a radical departure from the design pattern which had been established by earlier O.S. designs that we might reasonably suspect some degree of external influence at work. The most likely candidate is the legendary Bill Atwood, who had been directly involved with O.S. designer Shigeo Ogawa on the US marketing side since 1947. It would appear that Atwood persuaded Ogawa to produce a few sand-cast examples of a twin-stack .29 cuin. model whch Atwood evidently already had in mind, using some left-over Phantom Bullet open-frame timers which Atwood had lying around. Certainly, Atwood had a great deal to do with the later die-cast twin-stack glow-plug models - his name was even cast in relief onto the cases of the 1953 "New 36" twin-stack glow-plug model which was aimed primarily at the US market.

Information from Japan suggests that at most some 200 examples of this spark ignition model were produced, making it one of the rarest post-war O.S. products today. For a full discussion of the O.S. 29 twin-stack spark ignition model, see my in-depth article to be found elsewhere on this website.