The current parts do interchange with MX8 and all other square mono-block guns/receivers. The MX3 special ended production at about early 90s. The MX3 special block and the GA88 block are the same as MX15 an some others.
Hammer. I will not speak for current good doubles shooters, but .018 on the bottome and .028 on top will get you all the targets that you can point at when shooting doubles. Most would agree, this is a "Doubles Machine". Very well set up for dubs.