From the website:
M109 (155mm) (VUSBX08) includes three M109 (155mm) Self-propelled gun with M108 (105mm) option, four Vehicle commander figures, one M109 decal sheet & twelve rare earth magnets.
The M108 (105mm) and M109 (155mm) are the latest self-propelled artillery pieces in the US Army. Initially the lighter M108 was preferred for its ability to carry more ammunition and the relative ease of resupply. Later they were phased out in favour of the more effective M109, as the supply problems were reduced when operating from fire support bases.