Blancpain’s “Ladybird” Cal. R-550 was the smallest movement on the market when it was introduced in 1956. It would carry the manufacture through the 1970s in many ladies models and was among the last movements produced by the famed company before it was restarted in the 1980s.
The Ladybird movement measures 11.85 mm diameter and has a volume of just 425 cubic mm. Although hand made, it had tolerances of just 1/1000 mm and was fully jeweled. The tiny balance was quite heavy, with 22 gold screws for poise, and it boasted 40 hours of power reserve. Some sources show the diameter as 13.2 mm and list 33 gold screws. Perhaps this was true of later examples, but a 1957 Europa Star advertisement clearly states the numbers above.