As you have noted the 577 had maney diferant loads and case lenth. This is a sampling from Graeme Wrights book.
Original Cordite load 3-inch 100 grains 750 gn bullet 2050 fps 14.0 tpsi
Original Cordite load 3-inch 90 grains 650 gn bullet 1950 fps 14.0 tpsi
Original Cordite load 23/4 inch 90 grains 650 gn bullet 1950 fps 12.5 tpsi
I didn't see aney data for the 3 1/4 case but it should produse lower preshure.
Bill