The "Ingenieur" collection, whose name means "engineer" in German, was introduced in 1955 for engineers working in environments exposed to strong magnetic fields."Ref. 666" is a reference from that first generation. This is the "Ref. 666 ad," equipped with the "Cal. 8531"—the third generation of the "Cal. 85" series of movements featuring the highly efficient Pellaton automatic winding mechanism—and was manufactured from around 1959 to around 1967.Fish-shaped crown. Please note that while this particular piece was manufactured around the mid-1960s, the hands and dial were replaced in later years and now feature Luminova luminescent coating. Serviced by our shop. [Condition] The strap is in excellent condition with minimal signs of wear. There are a few minor scratches on the surface of the crystal. There is some slight corrosion on the hands and dial indices. As this is a vintage watch, there are other signs of aging and wear, but overall, it is in good condition for a vintage watch. 【Accessories】 Our store’s original box 【Size】 Men’s size case approx.: 36mm 【Materials】 Case: Stainless steel; Band: Calf leather by Accurate Form / Third-party stainless steel buckle; Crystal: Plastic