From what I was told the cal. 618 is almost impossible to find.
That is the opinion of 3 seasoned watchsmiths I talked to during my sourcing adventures.
The only one that has seen one of those was the eldest of the 3 and it was about 50 years ago on a vintage (even then!) Rolex Military pocket watch!
As far as I know, Cortebért Cal. 616 (15 jewels) mvmts were frequently "used" by Rolex and you can see them used by Panerai.
But those are also getting rare!!!
You can see reference of the 616s here:
http://www.knirim.de/xpane.pdf
Dont forget if you need someone to help you on upgrading your watch, or helping on your projects, Finepics will be the right man.
Cheers
M.