I live for the CABE
I definitely agree with the part about Schwinn shopping for different parts to save money. Schwinn also definitely wanted you(the customer) to have to come back to them for parts, which is why they had proprietary size tires made for them(by other companies). I agree with the logic of them wanting a front axle thread different than other makers for that same reason. My thoughts on the metric threaded axles/cones are that they don't make sense for an American company to make metric products at that time when everything else here was SAE(not implying 24 or 26tpi). The times we live in now is an entirely different story. It's still a mystery to me who made Schwinn's fore-wheel drum hubs too. They were a metric 9mm axle and odd size race different from their regular low flange counterparts. It's just the simplest explanation since Union's axle threads are the same as the Schwinn hubs and they are likely the makers. I have some 70s hubs stamped "Schwinn Approved Model 200 Made in Germany", they have Union Stamped dated axles. You're right about the possibility of an axle having been replaced after so many years of use....BUTWhy would you assume they would have used a 5/16 by 24 thread? They used a 26th on their cranks, not the normal 24th as found on other American build brands. They used unique tire sizes, etc. The Schwinn position was by using a finer thread pitch, it allowed for a finer bearing adjustment.
Showing a photo with an early dated Union made axle does not date the hub in question. Over the years Schwinn sold million's of replacement axle sets to dealerships that were used to sell over the parts counter and used in the service departments to repair their bicycles. The availability of any Schwinn service part needed was one of the Schwinn selling points. I'm NOT saying that the pictured axle was not original to your hub, there's just no way for us to know for sure today that the two parts were born together.
I have not ever read, heard, or seen documentation that Union built a Schwinn Deluxe hub.
If they did great. I'm just not yet convinced.
Replacement Schwinn axles were not dated. Replacement union axles I have found did not say Schwinn Approved. This 61 hub has 3 unique attributes, I'm not sure how else to explain that, other than it being the original axle to the hub, or someone putting it there just to screw with us, but which one is more likely?
Please, I'd like for anyone to offer their original examples up, or documents to prove my theory wrong or right. I went through about 4 Schwinn Reporters from 58 and 60(all I have close in months and years) and didn't see a mention of why hubs were marked "Approved."
Another side note, Schwinn did not document their change in banana seat pans in 1967 or 1970, but it's these little details that matter to folks trying to do a correct replacement or restoration. To Schwinn they were just another unit to bolt on and send the product out the door.