Thickness of the 'spacers' F and R = .180".
Thickness of the front mounting area = .314
Thickness of the rear mounting area = .309"
BUT the rear mating surfaces are flat, albeit the .005" difference. The difference must therefore be due to the outside surfaces not being level but this is academic anyway
Depth of the clearance recess in rear of base = .025"
Index plate screw hole moved to right approx .206". Which would make sense given that .180" + .025" = .205"!

For comparison, of the others, the average thickness of the bases at the mounting areas is rear .133.5 and front .132.5 (taken from 7)

Looking at the remainder there are other differences too. For example, those with a rear position quick release (Q-R) top-arm button in the base require that the body be drilled and tapped before the sight can be secured to it. This is catered for in the No8 rifle EMER I seem to remember.
While those with the front position top arm Q-R button utilise the existing ejector screw and backsight axis pin holes.

But that's not all........... Some of the front Q-R plunger types have a simple vernier scale while others have a dual range AND vernier scale. My head hurts already!