This is the largest of the yachting timers from Heuer that we have seen. It is in a sturdier case and has a screw on back cover. It dates from the second part of the sixties. The label on the back is surely the original one, but unfortunately it is unreadable. The enclosed page from the 1968 catalogue states the reference number to be 503.615 and the diameter to 59 mm. It is 18 mm thick.