Not 100% repeatable, but if you're working with symmetry planes on more than one ship at a time, and you spawn a new ship from the catalog, that you currently have symmetry plane on, while in a ship (regardless if that ship is the same one as the one you spawned - as long as another copy of it has symmetry planes), it will usually take on the symmetry planes of the ship you are currently in (as far as I can tell).