The Shimano Ultegra (6770-A) was an emergency stop-gap model that Shimano introduced around 2015. Shimano had been very happy to clear their stock of the 10-speed Shimano Ultegra (6770) as quickly as they could before it was made it obselete by the arrival of the 11-speed Shimano Ultegra (6870 SS). But then customers who had bought Ultegra (6770)s started to trash their derailleurs and wanted replacements. Shimano had nothing to offer them because they could not manufacture any more Ultegra (6770)s because it used a motor that was no longer made.
A legion of consumers of the Ultegra (6770) were furious at having bought a very expensive and prestigious derailleur system that was only a few years old but was no longer supported. To pacify these customers Shimano developed the Ultegra (6770-A SS), this combined the physical components of the Shimano Ultegra (6870 SS) with new electronics that made the derailleur a 10-speed rather than an 11-speed.
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