Now I don't mean to nitpick but because you seem to want to get technical, I will get equally technical.
Would you not agree that assembly by senior employees, hand fitting, and more inspections, actually in fact is not a physical difference?
Because while those things may be true, actual differences as in you pick up an 870P and then you pick up an 870 'Express', and attempt to quantify the differences, the differences are like 4 or 5 parts totaling about $30, and the finish of the metal.
(Furniture aside because you can of course put any furniture you want on any of them, so you could remove the fancy wood of a Wingmaster and put it on an Express or put synthetic cheap plastic stuff on a trap beauty Wingmaster or for example even better, the old style Police models had solid-ace wood with nice design and checkering and now Police and most tactical things of the world use synthetic 'blacktical' furniture).
I am not knocking the 870P in fact I want to make my 870 Tac a P model, but literally its like 4 parts and parkerizing, and poof. Identical.