We've seen these unusually lined bezels on Carreras a couple of times now, and there has been some debate over whether or not they were original. Personally, I was never convinced, as the 1/5 second bezel was an integral part of the 1st gen Carrera design so it would be very strange to produce watches without them. And now I think I have found the donor watch:

Rather a handsome thing in its own right in my opinion.