A grenade launcher was built into the roof of the Tiger from March 1944 onwards. This photo shows the device from the outside. When not in use, it was closed by a plug, as you can see here.
Photos of the roof in the Hachette kit seem to show weld material around the edge, but this didn't exist on the real tank because the launcher could rotate.
You could improve the Hachette Tiger Tank's accuracy by removing this weld.