Friedrich Franz II., Großherzog von Mecklenburg-Schwerin. Orchestrator of Mecklenburgian military integration into the Prussian army, afterwards officer and later general in the Prussian army during the period of the German wars of unification. Incidentally also held rank in the Imperial Russian army and was de iure the commander of the 8th Moscow Grenadiers, so that might be where confusion is coming from.