WotC describes these as Reserved Cards in the Official Reprint Policy
If they destroy the creature with counters, theres nothing left to put on the remaining creature
They are large enough for a few cards to fit comfortably but not too large that a loose toploader will move around a lot and rip the envelope
This trick works with all transforming double-faced artifacts, which in this deck includes Azors Gateway, Thaumatic Compass, Chalice of Life, Cryptolith Fragment, Mysterious Tome, and Primal Amulet, though if youre using Mysterious Tome, you should use that in place of the Incubator token instead of trying to transform it through this trick
The Ozolith Of course, counters decks did receive a major windfall in the form of The Ozolith