Rear hub looks like the original QD.
Rear wheel NS (drive side) wheel nut is horribly tight. Looking at the thread on the stud the threads look a bit "flattened",
cannot find any documentation saying what size
Fwiw, I've come across similar on Triumph QD hubs:-
. 9/16"-20 is a bog-standard Cycle thread; otoh, common "American" 9/16"UNF is 18 tpi, 9/16"UNEF is 24 tpi. There is a Unified 9/16"-20 but it's one of what's known as the Constant Pitch series; ime, it'd be weird for a 1960's wheel spindle thread ...?
. Despite the same tpi at several relatively-large diameters, Cycle and Unified have different thread profiles; ime, one squeezed into or on to the other causes the "a bit "flattened"" and "horribly tight" symptoms ...
die down it and a tap up the nut would sort it
Parts book says it's left-hand ... However, before you pour flammable liquid over the bike ... search out a pushbike repairer; curiously, even modern pushbikes use 9/16" Cycle threads on the pedal crank, right-hand on one side ... and left-hand on t'other.
Only thing that might rain on this particular parade is if the pushbike threads are 26 tpi (the other common Cycle pitch
Or Tracy Tools in GB has 9/16"-20 left-hand Cycle taps 'n' die
Unless you can find a competent machinist with a lathe that can screw-cut ...