Disk Shelf support rail part number needed

Guys,
I'm trying to mount a DS2246 disk shelf into a Netapp cabinet with a pair of
FAS8060 heads.  The shelf is coming from another Netapp cabinet and
uses the regular support rails.  But in the FAS8060 cabinet, the rear
rails are mounted further back, and it looks like there is an
extension bracket mounted on the rear post to support the support
rail.

I'm pretty sure my rails are as shown in this document:

   https://library.netapp.com/ecm/ecm_download_file/ECMP1139640

but I can't seem to find exactly what I need.  Some digging leads me
to think I might need X877B-R6 mounting kit.  Or better yet, is it the
X8783A-R6 with some sort of extension?  I'm across the country from
the system now, so I can't just go and look at it very easily.

Any suggestions are appreciated.

John