Hi, I do have here this S&G 6810, which I refurbished. It had a heavily rusted steel case and so I thought I can't do wrong. Perhaps someone can date this lock for me? Serial is 1306. The wheels do not have a movable fly and the dial has 50 numbers and the newest patent date of 1877. The company name is just Sargent & Greenleaf. Thanks

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PS: What is the purpose of this brass spring? Should it hold the locking bolt to not accidentally fall down?