Plug screws differ in having an o-ring or not (I don't think this affects damping characteristics) and they come with two or four drilled holes:

Given the two holes in the damper body I would have said that the O-ring provides a seal between the two halves of the system, so that fluid can only travel through the valve proper on the up-stroke and the down-stroke. Without that some fluid will be able to bypass the valve. Ed.

On outer springs I could find some information, they mainly differ in wire thickness and no. of windings (and colour), original MGB specification is the blue version with approximately 2.1mm wire thickness and approximately 5 1/3 windings. Some were built in together with washer(s), some not:

The inner valve part differs in no. of notches or cut-outs (1 or 2) at the top of the tapered part and the inner pin taper has two diameters, additionally there are also different springs:

You can see the outer cone of the valve with the green arrow indicating a ring of marking of the valve seat in the plug screw. The inner cone (red arrow) is in its seat in the outer cone. The "inner pin taper" is marked with the blue line, and the yellow arrow points to the notches or cut-outs in the outer cone. Some of my valves have two of these, some only one. I think, it affects the damping of the inner valve by letting more oil pass through when the valve is slightly open and even more when there are two of them:

I think these three differences are to give different damping rates, also the pin notches and small spring giving light damping with small movements, the main part of the valve giving more damping with faster movements, probably in one direction only. Ed.