the RJ48X was used on powered T'1 spans, the loop back feature was in the jack, it kept the repeaters alive, and for remote testing on a loop back with no cord plugged in.
RJ48C and RJ48X both use pins 1&2 for as rec from network, and 3&4 as xmit to network on T'1 lines
RJ48s used for all sub rate data 56K and down, pins 1&2 ,7&8