If a MEC comes up in 20 ga fairly inexpensively , I don't have a spare , you only need the sizer ring , start and finish crimp dies . For a 12 ga conversion you need the sizer ring , primer ram , and finish crimp . Also for either you'll need a shell holder .

Offered as a 12 or 20 will be easier to find .....

I passively looked for several years before a MEC 12 ga press and die set came along , it's an easy swap to do and MEC services almost everything on every press they have made since 197? .