Unfortunately, my copy of Gerald Kelver's Reloading Tools, Sights and Telescopes for Single Shot Rifles is packed away, so the following is my skimpy memory of his chapter (5, I think) on telescopic sight makers.

John Jamieson was a (Denver?) Colorado gunsmith who made a few externally adjustable sights in roughly the first quarter of the 20th century.  I own one, with mounts, and can attest that it is a very finely made and finished instrument.

Lawson C. Cummins of Montpelier, Vermont was the maker of the Duplex telescopic sight, the first such that was internally adjustable.  Based on advertising envelopes and secondary catalog references, Mr. Cummins seemed to have worked in the mid-to-late 1890s and maybe a little later.  In 40+ years of collecting, I seen only a couple of Duplex sights advertised and have never handled one.

Hope this helps.

Bill Lawrence

Gentlemen, thank you for your answers and help

Michael