I recall someone asking Lange years ago if the Saxonia AC movement was identical to the Langematik Perpetual movement except with 2 fewer parts due to the lack of a leap year mechanism. I believe Lange's answer was that the explanation is more complicated than that, which makes sense to me because the AC doesn't have (a) a leap year indicator, (b) a day-night indicator, or (c) the (very convenient) one-push all-function corrector button. I've always been curious how it is that there's only a 2-part difference between these two watches given that they share the same Sax-0-Mat movement but the Perpetual offers several more functions that would seem to involve more than just 2 additional parts.