Actually, the 55�º thread pattern is the British Whitworth form originally proposed in 1841 and subsequently adopted as the British Standard Whitworth. It was the best standard in Europe at the time and, as noted, used by several/most manufacturers. The American Unified Course standard uses the 60�º thread angle. Even the Luger grip screws are Whitworth pattern.
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If it's made after 1918...it's a reproduction
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