Guitar players have this weird thing where they want an item (amp, guitar, etc) to be "bone"/"dead" stock; BUT, they also want it to sound incredible (to their ears). It's a very weird phenomenon that's not common in other industries. For example, we don't say, "I had my kitchen remodeled and now my house lost value because it's not all-original anymore. I should return it to stock before I list it on the market." We usually look at those types of things as benefits that increase the value, not as "mods" that decrease the value. But with guitars, if I take the shitty Burstbuckers out of my Gibson Les Paul and put in a $500 pair of Lollar Imperials, the guitar actually loses value. It makes absolutely no sense.
This is especially tough with old Marshalls because Marshall used whatever components they had on hand at any given day. Fender did the same thing frequently too. (How many Fender Bassmans marked AA165 are actually AB165? Exactly.) So what is "stock" on an old Marshall where one day they were using 4700pf bright caps and the next day they weren't? What is "stock" when they were using 25k Mid and Presence pots one day and 50k the next? To that end, what is a "Plexi?" Is it just ones with a plexiglass panel? If so, is an early 90's 1959 reissue with a plexiglass panel a true Plexi? What about modern 1959HW reissues? Is it just 1959's with plexiglass panels or does it encompass 1987's? JTM45's with Plexi panels? Different circuit but still technically a "Plexi." Is my 1959 a Plexi even though it has a metal panel? You get the point.
So what is an "all original" old Marshall? What is "bone stock?" It's a bit of a pointless conversation. To me, if the circuit in an old Marshall is stock, then the amp is stock. The component values are all flexible because the original buyer could have gotten any variety of component values from the Marshall factory.