There was a Mk 20 UL that was aluminum (anodized with a black finish).
There was also a Mk 25 SA that was made from both stainless steel and aluminum.
There is also the Mk 25 T that is made from titanium.
The Mk 20UL was a study in the effects of saltwater on aluminum when combined with the dissimilar metals used in hose connections, etc. It also had different torque values throughout and in general was not overly durable. Very light, but not Sp's best idea by a long shot.
The Mk 25 SA was an attempt to create a Mk 25 version of the Mk 20 UL with the high stress/high corrosion areas beefed up with stainless steel. I am not sure what part of "dissimilar metal" they did not understand.
The Mk 25T is great and is very lightweight, but is not the best metal for use with nitrox and the whole reg costs about twice what any intelligent diver would pay for one. Nearly all the ones I have serviced are owned by people who are trying to flaunt their wealth. One diver even managed to maintain the price tag intact at the first annual service. It takes all kinds to make a world.
Personally, if I were buying a new reg, I'd go with the regular Mk 25 and not worry about it. If weight is an issue I'd go with the Mk 11. If I needed a cold water reg, I'd go with the Mk 17.
That said, the Mk 20 UL was not horrible, you just have to rinse it very carefully, dry it well, use the plastic isolators on the hose connections and lubricate the threads - and be very careful not to over torque anything. The US made black purge covered G500 was an excellent reg, but some techs have issues tuning them properly (requires a special tool and an additional step in the process.) The later Italian made grey purge covered G500's were not as stellar a performer for reasons I have never fully figured out, but they can also be upgraded to S600 status where they breathe almost as well as the older G500's.
