Extremely rare Swiss steel chronograph with register Circa 1940

Steel mono-rattrapante chronograph by Bovet
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An extremely rare Swiss steel chronograph with special function, so-called mono-rattrapante, and calibre Bovet, circa 1940.
Polshed stainless steel case, engraved; Unserem Bewährten Schwimmlehrer Eng. Fringer, 1925-1946, S.C.A. Original black dial with luminous digits, 30-minute register, bottom with dedication from 1946, Arabic radial hours, outer minutes/seconds track with Arabic five minutes/seconds numerals, subsidiary dial for the seconds, chronograph hand, and luminous hands. Extremely rare chronograph movement calibre Bovet based on the patent from 1936.
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Diameter: 48 mm
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Weight: 50 g
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Signed: No name
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Bovet patented this calibre in 1936, similar to a real rattrapante movement. In contrast to the 2nd hand, the mono rattrapante works so that the 2nd pusher is responsible for intermediate times. The movement was used for both wristwatches and, much less often, pocket watches.