According to the instructions, Grav-cannon assembly requires part 73/74 as the left arm for the SM. This is incorrect or a typo. 73/74 is also the left arm bit fort he multi-melta assembly and does not fit the grav-cannon assembly. If you take notice, the grav-cannon arm pictured in the instructions also has the thumb attached and is part 104/105. With the multi-melta the full hand is on the weapon. This allows you to build all the heavy weapons in the kit without sharing the arm.