I'd heard Schwinn made at least some EA3 rims (590mm) as well but have never come across one. You may be able to date the rim using the knurling in the center. There's a thread somewhere here about how the knurling on the S6 changed.
Schwinn's marketing was savvy with EA1/597mm. It was a size associated with British sporting/road bikes and club bikes prior to 27x1-1/4. Schwinn put its S6 designation on it, sold tires, and sold it as innovation. Their bicycles were very good, though their marketing was arguably even better.
I acquired three sets of unused S6 EA1/597mm rims from a closed-down bike shop awhile back. They were the 1970s era S6s. The welds were somewhat roughly finished. You could tell they had been rough ground to the point of making the rim usable but lacked a degree of fine sanding and final prep. Even in an unused state, if you put the rim on the truing stand, you will see slight side-to-side movement where the weld is. Even NOS, they won't run totally dead-straight because of the weld. Otherwise, they're decent enough rims. But they are certainly straight enough to build decent wheels and function with calipers. Final finishing on the British rims, I find, is generally better until you get up to the Schwinn S6 stainless, and then they are on a par with British. But S6 is what you want on many of the old Schwinn 3 speeds.
