Training Dummy Mk I: 4 iron plates
Mk II: 3 iron plates and a steel bar
When upgrading from Mk I to Mk II, the steel bar is not needed.. as the dummy is already 4 'units' big. The builder just build it to 4 and tada!.. dummy done. Steel bar still in inventory.
Upgrading from Mk II to Mk III gives same result. Only a steel bar OR a electric thingie is needed.
Also, when upgrading, if a toon is training, he won't stop and continues.. slashing air in front of him.
Hopefully, no toon was harmed during the upgrading..