Any second stage will work with any first stage if the second stage is adjusted for the IP of the first stage.
The only exception might he a Poseidon Cyklon since they have a very high IP.
But generally, the second stage doesn’t know what’s supplying it with the 135-140 psi and doesn’t really care, as long as it’s there.
MK/2, a very simple unbalanced bullet proof reg. Could be used for any basic recreational diving. Work of breathing (WOB) will gradually increase as tank pressure decreases (if the second stage is also unbalanced). Some people actually like this, as it provides kind of an early warning system that you’re running low on air.
I have one BTW and will never get rid of it. Dirt simple and super easy to service. Parts are only $10
If I was stuck some where super remote with limited resourced this would be the reg I’d choose.