When I see a 6-pin crossarm mounted over a 10-pin one in an early day telephone or telegraph openwire setting, I believe the six pin one came first and the 10 pin one was later added as the demand for additional phone service increased. Sometimes you will see slightly differing construction techniques such as the crossarm braces mounted on the outside of the top crossarm with the (newer) 10-pin arm with its braces mounted on the inside. Of course the top would have the oldest insulators, too ;-)