As I said, it is a measure of engine torque available and the 19600 in/lbs limiting torque is more about stress on the engine mounting structure than the load on the engine. The three models of the C130 (A-E-H) I operated all had the same Maximum Torque limitation despite different power outputs and propeller installations
ahoi !
so when the prop rpm was 1021 it would result that the herky had 1021x19600/5252 = 3810 shaft horse power per engine approved due to engine mount limitations- for all allison models.
the allison by itself would , depending on the model, give more but it was not approved, right ?
one more thing , but maybe i understand you not right : the 19600 in/lbs must be the value for the torque on the propeller, not the turbine by itself since a device which spins 13800rpm and produces simultany massive 19600 in/lb of torque would have an asronomic power ( 19600x13800/5252 = 51500 shp!!! )
cheers !