Briljant, I noticed that these are the parts for the complete undercarriage. I've created a list of the extra parts compared to the original set/undercarriage: https://rebrickable.com/users/jvdijk/lists/91872/
I would also be interested in this. Especially since I'm planning to modify the set that I already have with the modified boom mod and then build a second set with backhoe bucket. Would like to have both sets with the same undercarriage.