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The MK20 is essentially a MK25 without the ability to alter the IP from the outside.
Spot on. The Mk25 evolved from the Mk20 with the addition of an IP adjustment (sort of). Really, you don't get much adjustment; plus/minus one bar is about it. Major adjustment is still done by adding or removing shims inside the reg, but once it's set (in the factory), it doesn't shift by much.

It continues to evolve: the ambient pressure holes have gradually elongated, the piston is now a composite plastic affair, rather than brass and it also has a much sharper HP end and an anti-freeze teflon coating. The ambient pressure chamber is now ribbed, not for extra pleasure, but for improved anti-freeze characteristics (hence the Mk25AF name).

It is possible to buy the parts to convert a MK20 into a MK25.
Correct, but buying a Mk20 and all the parts you'd need to convert it to the latest Mk25 spec would cost more than buying a Mk25 to start with. The new piston is a very worthwhile upgrade and your service technician should swap it out for free during a service. I service my own and my LDS will swap an old brass piston for a new one for free.

I use Mk25AFs for my backgas and Mk20s for my deco regs.

I may try to gather together some images of the various generations of Mk20/25s to aid identification.

Watch this space.....
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