I think if it's between 5 and 10 mm difference it's not that a spring has been fitted wrong. If you have a look at the photo below there is far more than 10mm difference. Personally I don't think it would be possible to successfully mount the axle if one spring was reversed?
The top spring is the newer 4 leaf rear. When fitted to my '72 4/4 it re- centered the wheel perfectly in the arch - moving the axle back about 25mm.
![[Linked Image]](https://tm-img.com/images/2021/09/07/uj4fEBuGk0v8O1mGXVPbYBEWfrO7_fTzRGZVC0E8Icu-tkHWK0hmSwnHE1rSE_kua3IC0nEXJVcbmaimoixoztDGToTD3yAwtiD6zLly_oysi2XBhqVhjynAuer0K_i-ZKADQPD2-LP-tLWB5S3It7hySXO5zTbqMt8BX0naIuMzDpIYFixkLbfgHZ98uNEx8uj24xwM.md.jpg)