I've never seen one, but it looks interesting.  I wonder how much they cost new?  Not sure if it would help my old 6500 up the hills or not?  I've never had it stuck, but some hills give it a workout!  Here is the link to the parts catalog:

http://www.tuthill.com/admin/Documents/downloadFile.cfm?filename=D702909PC(JD67004).pdf

The link did not seem to work.  I will try again:

http://tinyurl.com/m8qrsj