557th Field Artillery Battalion
(Source: Bob Boberg, 557th FAM Bn)
Erector has a boom on back that opens like a jackknife to lock onto the missile after checkout, fueling and attachment of warhead.. Then it transports the missile to the launcher and "erects" it for firing.    
The soldier on the right is getting dressed for guard duty and has on his scarf with dice depicting 5-5-7.