Missed purpose

The body of the socket uses the 19 "dimension from fully and perfectly in tune and just so precisely between the mounting rails. As far as a classic 19" device ie. Since the connecting cable of the bar but on the side (!) And is not routed to the rear, come to this dimension to several more centimeters for the cable. That should play no role in L-shaped hole rails.
If the direct space behind the mounting recording but now not free, as some variable rack rails is the case because of the U-shape, the socket can not be inserted between the fastening strips. The cable prevents the assembly in these cases.