The Battlepacks Grille 17/21 was an German WW2 project to mount either the 21cm Mörser 18 (21cm Jericho) or the 17cm Kanone 18 (17.3cm Jericho) on the Tiger II Chassi. The big round circle on the back was where the artillery piece would be placed once deployed. The circle would be lowered on the ground and then the artillery piece would be winched back onto it (as seen in this video here).