Larry, Some years ago I owned a 'skeet' grade 16ga. solid rib model 12 that was both clearly marked 'skeet' on its frame and 2&9/16" on it's bbl. I shot the proper length Mexican Mfg. Remington 'Huntsman' ammo sold through K-marts at $38~$40/case [those were 20 box days, so 500 rounds in a case] until I couldn't get it any longer, then I let that gun go to a collector who had a lust I couldn't refuse. I doubt that ammunition was low pressure by any standard.
I agree w/your position that there wasn't any method available in those earlier times of measuring psi, rather it was interpretation and extrapolation and foggy, meaning some smoke & mirrors w/o any intent to defraud. It simply wasn't exact and sometimes got misstated as psi, which it clearly wasn't & resultantly misinterpreted.
