Proofing became very arduous indeed, when black powder was replaced with smokeless powder. With black powder, the charge could be increased by 20, 30, or 100%, but doing so with smokeless powders resulted in dismemberment for the gun itself. It appears the 'oiled' case was an effort to standardize and reduce the pressure excursions that happen with smokeless, especially the smokeless powders of 1900 through the 40's.

As to my guess on 100% more breech thrust when oiled, compared to a normal dry load, the 100% was just a guess, just as the proof house's guess at something over 30% for tapered case that's oiled.

That the straightened, minimal taper case shows little to no breech thrust is well known by anyone who has loaded extensively for these Improved cases. The straighter and longer the case, the lower the thrust. Our .375's have only .005" taper per size - only 75% taper of a 'normal' Improved case, which is about .0075" per size. How do I know the straight case developes so much less thrust - I know as everyone who has been a wildcat shooter for 30 years knows - through experience. I suspect you are not familiar with loading for Ackley Improved rounds.