Mounting with mountPoint node

The above is not a feature that comes with shapeBase-derived classes by default, so I decided to add it to a custom class I've written, Pickup. I just had a look in shapeBaseImage to see how it was handled there. It seemed to be using a matrix mountTransform that would hold the transform to get to the mount point from the object's centre. So I realy just copied the image code to my own class (based off Item). Here are the changes:

in PickupData::PickupData

mountTransform.identity();

Just initialising the matrix.

in PickupData::Preload

S32 node = shape->findNode("mountPoint"); //Find mountPoint node
   if (node != -1) {
      MatrixF total(1);
      do {
         MatrixF nmat;
         QuatF q;
         TSTransform::setMatrix(shape->defaultRotations[node].getQuatF(&q),shape->defaultTranslations[node],&nmat);
         total.mul(nmat);
         node = shape->nodes[node].parentIndex;
      }
      while(node != -1);
      total.inverse();
      mountTransform.mul(total);
   }

Copied the code from shapeBaseImage preload to find the mountPoint node.

in Pickup::registerLights

Point3F mountOffset;
   mDataBlock->mountTransform.getColumn(3,&mountOffset);
   mLight.mPos = getBoxCenter() + mountOffset;

Originally this read 'mLight.mPos = getBoxCenter();'

in Pickup::ProcessTick

if(isMounted())
   {
	  MatrixF mat;
      mMount.object->getMountTransform(mMount.node,&mat);
      mat.mul(mat,mDataBlock->mountTransform);
      Parent::setTransform(mat);
      Parent::setRenderTransform(mat);
   }

This if-clause was already there - mat.mul was all I added.

in Pickup::interpolateTick

if (isMounted()) {
      MatrixF mat;
      mMount.object->getRenderMountTransform(mMount.node,&mat);
      mat.mul(mat,mDataBlock->mountTransform);
      Parent::setRenderTransform(mat);
   }

Again, just added mat.mul.

Now, this code does work - however, the object is being mounted on a point that's twice as far away from the shape's origin as it's mountPoint should be. Here's a visual aid:
The red/green dot is Kork's mount0 point, where it should be mounted. Red/blue is the weapon's mountPoint node. Red/red is the weapon's origin (roughly).
I've tested with a normal shapeImage, and the mountPoint works as normal there. So what have I done wrong?