Yep you've got a bit of plastic deformation there. Not to worry mines the same albeit not as bad as yours. The nut head's nicely rounded and a bit rusty too, might be time to replace the axle. You can't complain- she's getting on a bit in years. :lol Like red98 I also do it by feel.
Treat everything in life the way a dog would- if you can't eat it or foc it, forget it.