I have always thought the Buffalo Bore load used A.A. 1680. Even with the long (2.125") .38-55 case there is just not enough room to push the 250 grain bullets to 2,000 fps in the 20" barrel. The Lee 250 grain bullet is seated .525" deep. This leaves a theoretical capacity of 31.5 grains of Alliant Reloder 7 and 31.0 grain of Hodgdon 4198. This leaves room for35.4 grains of A.A. 1680. Things don't quite work out like this in real life but close.
With the 250 grain Lee bullet Alliant Reloader 7 reaches maximum 34.0 grains and 1,880fps - I run out of powder capacity.
With the Lee bullet 25.0 grains of Alliant 2400 will top 1,900 fps from the 20"barrel.
28.5 grains of Hodgdon 4198 will do 1,925 fps or a little better depending on the day.
I run out of powder capacity before I reach to 2,000 fps goal with the 250 grain bullets.
Marshall Stanton used A.A. 1680 and with the weight forward advantage of the LBT style bullets 30.0 grain delivered 2,050 fps. Marshall is the only one I have seen post of consistently getting over 2,000 fps with the 250 grain bullet from the 20" barrels.
Back in the days when you could purchase them the 250grain factory loads would deliver a pretty consistent 1,850 fps. I never thought they were loaded to maximum pressure.
