According to the link below, the April 18 bout between Darsim Nanggala and Marco Tuhumury was for the Indonesian Boxing Association/Asosiasi Tinju Indonesia welterweight (not the light middleweight) title.
http://www.boxing-indonesia.com/2009/04 ... fight.html
In addition, the most recent IBA/ATI ratings show Nanggala as the IBA/ATI welterweight champion and Tuhumury as the IBA/ATI's no. 1 welterweight contender.