This crank was installed in the late 70s.

Hello,

We'd be happy to look into this for you but we can't be sure of the correct replacement with it still installed. If you could have your son remove your operator and send a few additional photos of the arms that attach to the internal window linkage, as well as a clear view of the bottom installation holes, that will help us narrow down your options. In the meantime, you're welcome to look through our selection here. Thank you! 

Thanks for getting back to us. Unfortunately, we are not able to make out the details of your operator with the arms still attached to the linkage. We'll need to see your assembly fully removed from the window to narrow down your options. 

Great, we were able to narrow down your options based on the additional photos you've provided. Look over the three crank pairs we've related on this page to see how the dimensions shown on our product pages compare to yours. Note that the distance between the installation holes and the diameter of the hole at the end of the link arm are especially important. 

Great, just let us know if you need anything else, and feel free to look through our return policy here in case you run into any trouble.