UDN user ask us to change the API for the material to remove confusion with the material ID.
Also apparently there is an issue with the skeletal mesh merge tool, the user did not give any repro step or any code that fix is issue. QA will need to find the issue with the skeletal merge tool and add the repro step.
see UDN post in the link section
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-62131 in the post.