FYI, It will take a good 10 to 15 days, or more, to dissolve the stem. The goal is to do it without harming any of the harder metals, and to that end alum solution works very slowly.
It still sounds like its either the stem or a stem extender to me. All that is generally included in the crown/stem assembly is crown, stem, and sometimes a spring and a stem extender. It could possibly be an adapter of some sort to correct the thread size? Maybe someone else can chime in who has this particular model.