I had the same issues a few weeks back when I got hold of a M670, it came with a UNINEG carrier but no masks. I looked everywhere for the SIXMA and SIVOMA but couldn't find them. In the end I got hold of a SIRIONEG 6x6 carrier with all the bits and it fitted perfectly.
I found this while researching....scroll the very bottom for the different carrier configurations - the naming conventions are a nightmare !
The sixneg isn't listed and after looking at it on ebay it seems be a different construction entirely from the 2 carriers I have.
hope this helps