Just so there is no confusion, All 1894 Marlin 44 magnum caliber rifles no matter if it is a "P" or "SBL" stainless or blue, long or short were given a twist rate of 1/38. The only exception would be a custom barrel install after market, not by the factory. The only difference with the barrels were micro-groove or Ballard rifling. The rumor that the micro-groove barrels had shallow grooves is false, they actually had deeper grooves than the Ballard and more of them. The only "P" models with a 1/20 twist were 444 model 444 caliber 18&1/2" rifles.
Not to derail the thread, but my 1894P is definitely a 1/20". If the OP's rifle had the same twist, I'd likely try making a deal onon it.