While you're waiting for Robert's reply (that is, someone who actually knows what they're talking about) I think KM/SP began using 1/2- instead of 1/3-horsepower motors at some point in the 1960's. Whether that made a difference, I'm not sure.
Our 1960 KM suds-locked all the time, especially (as Robert noted) with towel loads. As a kid, I would wait for the first spin to start, let it spin just a few seconds, then I'd lift the lid and let the excess water drain; I'd close the lid and away it'd go. It just couldn't get rid of the spun-out sudsy water fast enough to prevent suds lock on its own.
Our 1960 KM suds-locked all the time, especially (as Robert noted) with towel loads. As a kid, I would wait for the first spin to start, let it spin just a few seconds, then I'd lift the lid and let the excess water drain; I'd close the lid and away it'd go. It just couldn't get rid of the spun-out sudsy water fast enough to prevent suds lock on its own.