I seem to remember that some of the -4 were round and some square in barrels 4" and over. The 2 1/2 & 3" barrels have always been round butt to my recollection.
The CS guns as well as the 2 1/2's have always been round and they date way back. The final transition was made during the -4 phase: early ones were square while later ones were round in all barrels.