I think it is based on Bone names and case sensitive, afterall almost every model has a Bone_Chest and Bone_Head. I also tested and Bone_Turret worked too. Guess you just have to use the EXACT bone name
I have done testing with this before for my vehicle systems. Only the bone_head and bone_waist work. (Bone_turret works, but then bone_head no longer works) GUI and JASS both do not accept any other bone names. You must use a bone name, not an attachment.