Danish Tambour Door Credenza with six legs - Svend Larsen?  

Since there is no apron base to which the legs attach, I am going to assume the legs screw directly into the bottom of the carcass.
So first I would unscrew all the legs and check that they are all identical in shape, wood species, quality of finish, threaded bolt, and especially tool marks from the lathe. And I would check to see if the threaded inserts in the center of the carcass match the outer ones.
If there are differnces, I would conclude that a middle set of legs was retrofitted to correct a sag. If not it could be that the factory came to the realization that another set of legs was necessary to avoid a sag.