My understanding is similar to @bondmanp but the variance I recall being mentioned I believe by John Strohbeen, maybe somewhere in his blog on OHM site, is version to version over the years more so than model to model. Not sure. A call or email to Ohm would probably provide a more concrete answer. Would be interested in the answer. I think I recall aluminum being mentioned recently with at least some latest versions as well. Mine are one generation older....series 3, pretty sure they are not aluminum. THere are 4 generations/versions over the years since the early 80’s: original usually designated X, series 2 and series 3 usually designated X00 and current line designated X000.