The 4.5 and 5.5 stock / pin / trunnions also differ in angle of stock to reciever. Mixing the two makes for very odd and unwieldy guns. 4.5 is more common, 5.5 is Russian clone bone correct. Everyone else uses 4.5mm.
The milled AK was a stopgap to figure out the forging, heat treating, and riveting on the new system in 1948/9. Once that was ironed out, milled went away as it is much more expensive and heavy.
AR guys looking to get into AKs and having the Lego mindset are in for a rude awakening, as nothing fits out of the box, and there's a dozen or more countries of origin that sometimes all use their own measurements. Hell, sometimes the same country part doesn't fit its own country's gun. We're talking drunk peasants making these parts half the time.