The 2 semi-tubular rivets holding the pieces together are normally steel with zinc plating.
After I drilled out the rivets I checked the material with the old magnet test, they were not attracted, eg - aluminum. That was a couple of years back. Still, I checked a couple more housings in the TBD stack, five to be exact, and on four the magnet stuck, one would not hold the magnet, aluminum. Sources are unknown. The date code on the odd one is F6, June 1966, last of the 66 year. I believe that the steel rivet is more appropriate as aluminum and steel do not get along in a wet environment.
Jim