With no clip behind the upper arm pivot, it could be from as late as 1972, The chromed bronze arms are not typical, long gone by then, but the first generation mechanisms also used slotted screws to hold the parts together, too bad they stopped doing that, as my first unit (1972) almost pulled itself apart long ago.
The early unit I have in addition to the screws at the pivots has no slot at the cable stop extension, but a captive chromed barrel to accept the cable, a surprise find on an ebay bike a while back. Also for reference the cable fixing bolt has a washer shape machined as part of the bolt, not a separate washer as seen later. That one could be a "first" generation designed unit.
John Jorgensen
Palso Verdes Ca