The original DSR was indexed under the handguard, and attached at 20 ft/lbs using a 1" 16-point socket.
At that time, KAC did not produce a low backpressure DSR suppressor, so the 14.5" barrels were ported for 100% suppressed use.
As the program developed, it became clear that an indexed rifle suppressor was not going to work out, and there were developments in both the guns and suppressors that showed that we could get there without some things that were assumed to be needed.
So the program approach shifted to a standard 14.5" upper with a direct-thread suppressor that permitted install and removal with a standard tool, off of the rear of the suppressor rather than from the front.