This large watch was made by Longines specifically as a high-precision timepiece to be submitted for observatory testing. As confirmed by the Extract from the Archives, the ébauche is a Longines chronograph caliber 260 but without the chronograph mechanism fitted so that only the going train is present. It is likely that it was made this way because a large-size movement may contribute in itself to a more stable running rate. It is fitted with a Guillaume balance, regarded as the optimum balance to eliminate errors arising from fluctuations in temperature.
Charles Edouard Guillaume invented two nickel-steel alloys he named “invar” and “elinvar”. Invar has a near-zero coefficient of thermal expansion, making it useful in constructing parts whose dimensions need to remain constant in spite of varying temperature. Elinvar has a near-zero thermal coefficient of the modulus of elasticity, making it useful for springs that need to be unaffected by varying temperature. Elinvar is also non-magnetic, which is a secondary useful property for the making of watch springs. Most competition chronometers made by Longines used the smaller oblong caliber 360 making the present watch unusual among them.
Submitted for testing at the Observatory of Neuchâtel between December 1969 and February 1970 where it was awarded a Bulletin de Marche recording an average running variation of +- 0.07 seconds, coefficient thermique +0.018, Erreur secondaire de la compensation -0.31, reprise de marche -0.81 and positions +-0.07.