Incidentally, the brother has only recently found this D6C, the very first Cat, and the first new bulldozer we ever bought (out of about a dozen dozers in total), and it belongs to an old prospector in the W.A. Goldfields, and it's still fully operational!
It's probably done around 50,000 or 60,000 hours by now. We bought this tractor new in November 1966, and sold it in 1972 (traded on a new D7F), and it had done over 13,000 hrs, back in 1972.