A mobile crane having a boom and a boom extension connected thereto with a toggle linkage for connecting the boom extension to the boom to allow the boom extension to be offset from the boom by one of a plurality of predetermined angles. A pair of double links is pivotably attached to the boom extension while a pair of single links is pivotably attached to the upper sheave pin in the boom head. Both the single and double links have a hole in the opposite end. Either the single or double links also have a hole in at least one intermediate position. The link having one hole is then pinned to the other link having two holes, the hole selected in the second link determining the angles of offset when tension is applied to the linkage by pivoting the boom extension about the lower sheave pin after removal of the upper boom foot pins. Biasing devices are provided to cause the linkage, which pivots freely at both ends and in the middle, to toggle upward when tension thereon is released. A storage position is provided for storing the double links and the link connector pins so as to prevent interference with the other apparatus and loss of the connector pins when the linkage is not in use.