Anyone who has had a child knows that the bellybutton creating process is nearly 100% passive. The cord is tied off in 2 places and cut between them. After that it just dries up and falls off, and whatever is left is the bellybutton, be it an innie or an outie. Today we use metal clamps, and in "ye olden days" they would have used strips of cloth or string, but otherwise the process would have been exactly the same other than the fact that today we would also apply an antibacterial substance to prevent infection. So there is no reason to believe that bellybuttons of past eras would look any different than bellybuttons today.

You can look at sculptures of the human form from very far in the past if you want to see what belly buttons looked like. The answer is that they are virtually identical.