I’ve tried researching this independently but I’m coming up short. I know the technical differences (18 jewels in the 1861 vs 17 jewels; rhodium plated in the 1861). But why did they make these alterations?
Good question. The single jewel that was added replaced one of two bushings in the coupling yoke. If this was a concern for wear in the bushing, then they would have replaced both bushings with jewels, but only the visible bushing was replaced with a jewel. In all the hundreds of Speedmasters I've serviced, I've never had to replace a worn parts in this area... I can only conclude it's marketing. Cheers, Al