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I believe we are describing the same type, but while others may have made it over an extended period, Wade and Butcher's version, if the SRP Wiki is to be believed, was made between 1830 and 1837:
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I could be wrong, but it looks to me as if the wedge pin has a collar too. It is hard to see, and one of those 60x illuminated Chinese magnifiers which sell for a few dollars on eBay can be a very...