I, too, have a pre-2005 KitchenAid 600 Series Professional mixer and the same thing happened to me yesterday. There is nothing inside the mixer that seems to hold the planetary in. The top of the planetary has a lot of little grooves that appear to possibly hold the head in by friction. After spending some time finding a place for it to easily slip in (testing fit as I slowly turned it) I found a place where it slipped in easily. Before I rocked it in, I applied food-safe grease to all the gears. I then pushed it in and gave it a few taps with a rubber mallet to make sure it was seated. It seems to be working fine, but I will update if something else happens.