The choke should be marked on the barrel near the receiver. Look on the left side. If you can't find it measure the inside diameter of the muzzle with a caliper if you have one and let me know what it is.
The gun was manufactured in 1964.
There will be *'s on the left side of the barrel for the choke, let me know how many or what other markings are there such as **$, *-, **